|- Energy, Power (calorific, mecanical, kinetic, chimical, nuclear) - Mass and inertia (what is the mass; the force, the fire retained by the mass, thickness) - mass/density (nature of matters ; mass and weight, a dilemma ; a coin og aluminium that flot) - mass and energy (dinosaure and rocket) - Forces/balance (conditions, game) - Forces/centrifugal (our universe ; everything that spin.. ) - Forces/direction/composition (application des forces, hamac, wheelbarrow, coupola and architecture) - Force/ resistance/ reaction (the matter oppose a resistance action, reaction) - Acceleration, speed - Heat - Cold (surprising effects of the heat and cold) - Pressure-vacuum (liquids and rocks ; the delphin and the birds; the paint that runs, the ice that melts) - atmosphéric pressure (we are under it ; the content that doesn't get out from the upside down can ; atmosphere and pressurisation (planes) ; meteo, vacuum, seringue.|
The bottle in balance on a coin. TRICK !! Let us start with the empty bottle, usually of sparkling champagne, because it is heavier and it is the festival !! Place a coin under the top of the bottle which naturally is upside down and in balance on the coin. Ask somebody to remove the coin without touching the bottle (it is impossible) solution : take a knife with a long flat blade (regular, kitchen) maintain the blade on the table and in a firm and swift movement, drive out the coin with the blade. The bottle almost does not move because of the inertia of this one.
What is inertia ? inertia relate to the alteration of a movement : low speed of a cargo liner, but which cannot change its road nor to stop due to its inertia (heavy mass). Inertia is used to regulate (the plate of professional pickup weight several kg if not tens of kg ! ), to absorb a chock, a vibration. Without weight (mass), there is no inertia.
Let us take again about satellites : despite of their speed compared to the ground, two satellites side by side do not have any energy between them but they have inertia due to their mass and if they knock each other, even at very low speed, that can made big damages (low speed but mass, to simplify, is significant).
The cosmonaut in his cabin must pay great attention not to be wounded, because he is floating but has always the same mass and can hurt himself if he knocks against something.
Mass effects : effects of mass and inertia can be understood differently, for example when burning or damaging something :
Understand the mass ; some exemples
Mass and fire : the fire which lights the touch wood but not a log. Prehistoric men would have used a kind of touch wood (amadou) prepared from these large mushrooms which one can sees on the trunk of the trees ; a spark resulting from a (particular) stone shock was enough to ignite this matter extremely fine and light but a spark will not put fire at a oak tree trunk, massive and dense ; curtains and bed covers, ignited by candles cigarettes or a bad electricity wiring are often at the origin of a starting fire, with oil in the kitchen and petrol for the barbecue.
Information of an insurance:
One advises not to put hot or tepid products especially if their weight is significant; two tepid coffee cups to prepare a coffee ice creams will not have any incidence if the refrigerator or the freezer is rather loaded. Thy can add somme moisture if not sealed with a cap.
To keep a good mass cooled inside the freezer if it is not full, put water bottles inside (you will increase inertia). It is the same reasoning for the refrigerator section. If you have to receive guests, you will have a good idea, if it is not to much full, by forcing it before during a few hours and by putting two or three water bottles which you will leave at the beginning with the new food.
Penguins in Antarctica gather in huge number to fight against the very low temperature, the mass itself making a movement that conduct inside penguins to
outside edge with a rule we do not know ; it seems that overheat sometimes in the centre, but may be it is a mere gap of fresh air ? (my interpretation).
Mass and moisture. A depression in the ground protect a bit the soil from drying too much (culture in dry country). I remember this fact in some part of Yugoslavia where vegetables were growing in nearly individual little bowls, dug in the soil. Pots and lawn : tight gather your pots in the garden in summer when leaving it for some long vacation. And make a hole ! .. or put some protection against the sun; the "molehill" of your lawn will dry faster than the parts in hollow (it is also due to the angle of the sun rays).
EMERGENCY : to make a door resist to the fire, soak it constantly with water if you can.
Plate of restaurant : all slips or is spilt. The inertia of the dishes and liquids put on the plate makes them slip or spill when one moves the plate too quickly. A waiter tilt the plate a bit and the forces are exerted perpendicularly to the plate so that everything keeps its place. See centrifugal force below.
AMAZING: Narrated in a film by Paul Emile Victor, here's a curious cook imagined by people of the forest at that time. Hollow bamboo are maintained banked the soil, which are strung in long rolled leaves containing food. The bamboos are filled with water, then a fire is lit underneath. And the water boils gently until cooked.
why don't the bamboos do not get burned ? As long as there is water in it, a bambou is protected. Drawing by "hibiscustour.
Moment of inertia. the cosmonaut who swivels and the cat which always falls down on its feet.
it is always a problem of mass and inertia ; one speaks about "moment of inertia" a term which recalls the leverage with which one exerts a force. A body floating in the space does not have any possibility to turn since it does not have any support. The force which moves it "grasp" it by its centre of gravity", which is in the middle of it if it is a homogeneous ball or some other part elsewhere, close to the heaviest part, if it is another case. What can make the cosmonaut and the parachutist to spin or the cat to fall down on its legs? only one possibility, in modifying the spot of its centre of gravity, i.e by modifying the shape of its body ; how? for example by folding the legs and stretching its harms. A similar case occur with the skaters who raise the arms and tighten the legs to accelerate their rotation : there is formal alteration of the body's shape and as acquired energy must be preserved, it is the spin speed which increases automatically to keep the same amount of energy (body is thinner).
The supermarket trolley the moment of inertia appears in our day to day life, for example with the trolley of supermarket : put what is heaviest and dense the closest to you (drinks.), while piling up if possible : your carriage will be more operating, when you turn, that if the mass is at the end of the carriage (the greater moment of inertia, the larger effort). Of course, straight ahead, that does not change anything. One can even more reduce the moment of inertia while plating oneself against the bar and by grasping the carriage by its sides, arms half stretched.
The bicycle with comparable weight, the large wheels are more stabilizing than small because the moment of inertia is more significant (gyroscopic effect); to reach a comparable effect, the small wheels must turn much more quickly, because the effect increases with the square of the ray.
The gyroscope as for the wheel of bicycle, it is the centrifugal force associated at the moment of inertia which gives to the gyroscope its astonishing stability (always used on the aircraft and rockets, possibly with an aadditional GPS). To obtain a significant effect of inertia with a very small "wheel" diameter , it is necessary to make spin the gyroscope at very high speed (around 100.000 rev mn, from where problems of axes). Three gyroscopes, suspended without friction, always indicate the same direction independently of the movements of the plane.
Mass and density.
a body is denser than another if, with an equal volume, it is heavier ; all housewives know that because they often weigh up to check what is sold to them.
The ratio (division), between weight and volume gives the density. The water, taken as reference, has a density of 1
(one decimetre cubic, that is to say three sides of 10 cm, weighs 1 kilo (*).
The wood generally floats but there is one which sinks (heavier then water) ! density :
(*) true for pure water. The salted sea water is denser and one floats better on ; the Dead Sea being by far more salted, due to evaporation : 250 grams/liter, it is brine ! (closed sea, the lower point of the surface of our planet : less 390 m) ); both Red Sea (40 gr/litre) and Mediterranean sea (38 gr /litre) themselves are more salted than the oceans (35 gr /litre) which are themselves a bit different from the time when "the first living being" left them to live its own life on the ground, 400 million years ago.
(*) ; atolls have very often a very salted water, which slows down the animal life (some of them raised at a certain time, without any communication with the ocean). But there is pure water of greater density, called "heavy water" because made up of several basic molecules (H2O) agglomerated, which was a raison of battles in Norway during the last world war (for atomic energy). Let us note that the number of electrons is not enough to define a body ; iridium (77 electrons) is an extremely hard metal whereas mercury (80 electrons) is nearly liquid.
Surprising effects of the mass of a liquid (floating) : close to Frejus, the old dam Malpasset shows concrete enormous blocks and of riprap which where transported on several hundred meters during its
rupture. It is now a beautiful walk, walking the upper path (scrubland) or the lower one : one always wonders why such
elements can be carried away by water in fury, are they trees, sides of mountain, cars, houses ; the answer is
always "the incredible power of water"; it is true but very insufficient, because it is especially a problem of density
: in water, the concrete, the stone, the car (hollow), the tree do not weigh almost anything or anything (see above density
of the stone). Thus actually, water carries only light elements for itself, also solid masses are they, which are
easily following its course !
As for the radioactive elements, whose residues are common, their density and hardness is alas used to perforate the shieldings, with the terrible consequences which one knows (wars of Iraq), on soldiers and people, not informed (serious illnesses, montruous children).
PRACTICAL LIFE, the melon ; The housewives weigh up melon to know if it will be good; it is indeed one of the elements to be taken into account because if it is heavier than another with equal volume, there are chances that it is more sweetened.
Mass and energy The jump of the kangaroo, the elephant and the flea.. the starting of the large powerful car and the bicycle.
To jump high and far, the kangaroo "is a sort of big thigh", with a small body; it was the same for some dinosaurs which, in spite of the hugeness of their back feet could not jump like a kangaroo. One verify well with these examples, the limits of the muscular power for an consequent mass (weight); to make an elephant jumping like a kangaroo, imagine the thighs, which would perhaps make at least 10 tons each because the weight of the muscles increases with the cube of dimensions and one reach quickly at an impossibility : the muscles could not make jumping the muscle weight plus the body, say around 23 tons. With the flea, very light, its muscles are very powerful compared to the total weight (good ratio power/ weight) and it jumps very far and very high for its size. And with the car very powerful, but very heavy, it can be beaten starting by the bicycle.. on a few meters due to the inertia of the car.
To concretize the example of the monstrous elephant, which can lead to smiling, here an example which will convince you : that of the rocket "Jupiter C" which launched the first American satellite in the space, on January 31, 1958, after the exploit of the Russian "Spoutnic", then a satellised dog., the 22 meters high rocket weighs 28 tons. (*) the fuel constitutes the essence of its weight: the satellite, is only 65 centimetres length and weighs only 8 kg. The 25 tons of fuel of the first stage will be exhausted in 2 minutes and 25 seconds. I think that you already understood the link together with my elephant, because the rocket "is a giant thigh" which launch a very small body, with a big difference because the grease of the thighs of the elephant will never melt at this speed (it is the fuel) and in any case, its energetic capacity can be measured with that of the rocket fuel which is an extremely dangerous gas paralysing, reaching 2500 degree while burning !! A new concept thus intervenes, that of the power/weight ratio (very weak in the living beingss compared to chemical energies of combustion (like engine of cars, said "to explosion", gas turbines, powders, rockets). Let us keep in mind that in these cases, there is a very huge loss of matter in a second.
(*) By way of comparison, the first ARIANE rocket, which successful launching was December 23, 1979, weighed 210 tons (160 for the first stage), for a mass put
into orbit of 2 tons (third stage, is approximately 100 times less). Fuel: hydrazine, tetraoxyde of nitrogen for its four engines of the third stage (booster rocket). To be more
precise, it would be necessary to also compare the height of the orbits, but I do not know it.
Note ; the launching of many satellites is largely facilitated if one launches them close to the equator, as on our French/European basis of Kourou (Guyana) because one benefits from the rotation of the Earth which is null at the poles and maximum at the equator (approximately 40.000 km per 24 hours is 1600 km/h !). One understands better than the Russians were kin on joining Europe for launching its "Soyuz" to Kourou instead to build expensive and random floating base as they had considered. -
One also refers to potential energy and kinetic energy :
- The potential energy (of position, possible) is that which could be spent, as for the lake dam, the stock of powder or gas of the rocket, gasoline of the tank. In the case of the dam, it is the height of the drop (average between the two levels of beginning and end) and the forces coming from the mass (*) of the dropping water which will determine the value of energy (E = F (exerted force) multiplied by D (the distance from displacement of this force : height for the dam or distance for a vehicle).
- The kinetic energy is that which is spent (when the water flows down). Energy kinetic refers to mass and speed, which will be addressed below (see the pressure). Kinetic energy is expressed by a simple formula :
unit : a Joule spent during a second deliver a power of 1 Watt (one knows better). Formula
Coming back to the dam, the potential energy will be equal to the volume of water released
Let us reconsider gravity
If one wants to see large and far away, one can to say that centrifugal force and its opposite, the entripetal force (that which "retains"), relate to mass of celestial body and their revolving movement, the whole regulating the universe and certainly our life (we hold on ground only by his centripetal force, the gravity). One says sometimes wrongly centrifuges for centripetal, force which equal and is opposed to it, but it is of no importance, one understands oneself).
420 million years ago, the moon was very low at that time, only 150.000 kilometers from the earth, so to speak half of the current distance, and turned three times more quickly around the earth (ration between enrgy, rotation speed and revolving distance).
Let us return on ground where the first men quickly understood the action of the centrifugal force : the sling is initially made up of a string and a small case which allows to make turn a stone, accelerating it until its release at the right time to launch it enough far. It should be noted that the launching of the hammer, at the Olympic Games, proceeds of the same technique. We note that the weight does not escape from a radius but from a tangent of the circle
On the other hand the "launchers of sagaie", appeared 20 thousand years ago, increase the levering and thus the speed of launching, which could reach 50 metres/second (sagaie of 2 meters).
Satellites, it is the centrifugal force which tends to make it escape from the earth, struggling with the centripetal force (gravity) which tends to make them fall down. When both are equal (in balance), the satellite would be stabilised on a circle. This is purely theorical and never got. Reality is that after the launching, the satellite begin to fall down to the ground, revolving between a highest circle ad shortest one (ellipse).
It is the centrifugal force which makes hold the spinning top upright, but one observe that it moves also rotating aside (movement of precession).
The wheels The centrifugal force increases with the radius, and for this rule a wheel of bicycle, in spite of its light weight, has a great inertia (due to the centrifugal force) when one keep it rotating while holding the axes in hands; a large wheel gives much more stability than a small one, even if this one is heavier. Large wheels provide also speed regulation, as it was for the heavy phonograph plates. To the wheels and spinning tops, it is necessary to associate all that dries : from salad to the washing machine, while passing by attraction parks, the extraction by centrifugation (greases of milk, honey of the plates of the hive, sorting of the coins, medical preparations (separation of virus) and so much of others..
The gyroscope. It is still the centrifugal force which allows to directs planes and fused nowadays, in addition to the GPS and not in replacement (for reasons of independence and safety). In this case, the diameter of the "wheels" being very tiny, one makes them turn at very high speed to compensate (around 100.000 revs/mn). These wheels (gyroscopes) indicate the direction independently of the movements of the plane (there are three of them) for more precision.
The turn in the car, the pendular train : the banked curve allows the car and the passengers to be in balance between gravity and the centrifugal force, but for only one speed value ; the same for the "pendular" train whose cars are inclined in the turns, as for the plane whose radius of the "standard turn" is limited to avoid a too great inclination (and thus a felt compression "G" too significant).
Harmful effects of the centrifugal force: the centrifugal force is not always opposed to centripetal force, as in the universe, for the wheel or the sling ; sometimes it is simply the contact, friction on the ground (car in a turn) or the resistance of the matter (the grinding stone) which resists and if the threshold is exceeded, "the cord breaks" i.e. that the car escapes from the turn or that the grinding stone explodes (or the abrasive disc of my grinding machine, enough dangerous).
The effect of the centrifugal force is also conditioned by the "moment of inertia" (see in complement "masses and inertia", see Mass and inertia, (bas)
The roll of paper, the door that wedge, the terminal stack, the whellbarrow, hypermarket trolley. , the boat, the plane (slope, stream, wind), the hammock, the cupola.
The roll of paper. It is a simple example. A roll of paper is hanging by an iron shaped in a horizontal U. A branch of the U is screwed to the wall and hinge in a, while the other branch goes through the center of the paper roll. Pulling down the paper with a force p, can make wedging the roll against the wall (*). This is because we don't pull in the direction of the axe a. Consequently, another force m push the roll against the wall and squeeze it. (*) if the point m dont slip well (case of a wide roll, rough or vinyl wall.
Conclusion : On can pull the paper horizontaly or up but it is better to put the roll with the sheet hanging against the wall. Pulling the paper will then pull the roll away from the wall.
The door jammed. A door is rubbing and edging on the floor and you push it on the handle. Doing so will deform it slightly and it is wedging even more. You notice that shaking it back and forth, makes going a little better. First case, the door opens by pushing it : it is better to push the door at the lower part, near the floor, exerting so a horizontal force as closely as possible to the friction; for that, use the foot or the calf and "relieve" the door slightly upward while pushing with the handle. Second case, the door opens by pulling it : it is less favorable because we can only "relieve" the door up slightly while pulling with the handle. See "Brico 1" to reduce or eliminate the problem.
Angle or terminal fence stack/piquet/pole. the wire that is stretched (h) exerts several forces described on the drawing. The vertical pole (Vh) tend to go up (get out of the soil), due to the leg (j) at an angle, that itself tend to go down. Click on the drawing : translation French-English for the drawing : ancrage=anchor, ciment=cement, pierres=stones, parpaing=breeze block.
The wheelbarrow (click on the drawing); the wheelbarrow offers also a good example of forces composition : the force exerted while pushing is also directed to the bas since the handles are higher than the axis of the wheel (even if you have long arms and small legs..); The exerted force can be transposed into two forces, one in the direction of displacement (fortunately forward), the other downwards. This last force is cancelled by the resistance of the ground if it is rather hard, but a stone or soil depression will tend to make "plug down" the wheelbarrow. The best solution is to U-turn the wheelbarrow and draw it; the second force is now directed upwards and the wheel jump much more easily the obstacle.
The slope, the stream, the wind.
The trolley drifts on the slope, the plane drifts in the wind and the boat drifts in the stream : all of them go diagonally to compensate for inopportune side displacement. Consequently, the trolley, the plane and the boat always go at a certain speed on their axis, but at a lower speed in the trajectory they be follow Click on the graphic.
With the large planes "flying across" the pilots can find themselves skewed over the lawn, while the main wheel train is on the track!! A large carrier of 250 to 550 tons does not handle itself as a wheelbarrow.
In flight, with a speed of 850 km/h for a Jet, the same wind will naturally have less effects (speed and angle) because on the graph, the V200 arrow will be much longer (measure in proportion 850 instead of 200). It will still fly skewed to maintain its course.
Boats; the sailing boats slip on the water without plunging down nor to bouncing like motor boats do and especially outboard motor boat. This is due to the forces to which they are subjected. To simplify, there are both the resistance to run forwards related to the friction of the hull on the water and on the force of the blowing wind in the veils. However this force being above, it makes plunge the boat forwards. It thus tends to plough inside the waves and navigation is very stable, though there is a little banking on the side. With the motor boat, especially the outboard motor boat, the force of propulsion is now under water, i.e. slightly below the force of friction of the hull and it is the reverse which occurs, the propulsion made rock the boat backwards and so the least wave make it bouncing as soon as one puts a little power: the boat then makes a succession of rebounds while slamming downheavily.
The hammock (also the clothes line, telephone/ electric wire.. ); the hammock offer a beautiful example of forces breaking, although a little more complex than for the
One builds what it is said the parallelogram of the forces, G, t'1, t'2 and (opposite of the weight), by tracing lines which are parallel.
By reducing the angle to zero (impossible, but it is interesting to think at it), the force becomes.. infinite !
This example shows that wires and cables can never be tightened completely because their own weight prohibits it and in spite of appearance, they most often are strongly tightened !
What's more, the winter temperatures tighten metallic cables a bit more, and the snow and ice can break them. Calculation is more complex because the weight is distributed all the length and the curve known as "of the chain" is solved by a calculation of successive approaches (iterative).
The cupola ; let us take again the example of the hammock, but with some modifications : it is supposed that the drawing is reversed up down and that the hammock is a curved supple board. If one sit down on the board, one see now that the forces which drew on the posts are now forces which push them. What interest ? that of a stone roof, as in the cathedrals and mosques ; this one, by its own weight strongly pushes the walls outside and they must be reinforced. To resist, Roman churches have large thick walls and reinforcements by low walls as well as small openings. Gothic art, from the 11th century, which lets enter the light by both many and broad openings, called upon the low walls but also with the jambs, to prevent the walls from collapsing. Our Notre Dame de Paris offer superb ones and they are not decorations. Without them, the stone roofs would make rock the walls towards outside. See "roman and gothic" roman church".
Bar, gin hoist, lever, wedge, material wet, screw, pulley, hydraulics
The bar carried by two people or more, makes it possible to divide the weight by two at each end. A large handle or several small brush handles bond together makes it possible to raise an heavy object which one wrap with cords or which one makes rock on a cloth which will be then tied around the handle. One can also think at rollers which release the weight if the surface is rather hard and smoth ; one uses wood logs placed under the object to move ; in the garden, it is necessary to put planks. The rollers freed at the back are put in front of : it is a thousand-year-old method.
The gin hoist is an assembly of two posts bond at their ends so that they form a V upside down (like our two legs) ; one fixes bonds at the top of which some will be used to wrap the loading lying on the ground and others, longer which will be used to draw the gin hoist. The load (heavy stone), is bond short so that the posts are in skew. While pulling the long cords, the goat rectifies itself and finally tilt on the other side, raising the stone and shifting it a little further. People think that the Ancient used this process ; for instance about the displacement of stone and statues in Egypt ; in the Easter Island, one have discover since some tow path tug may be for logs (One think that this island had been ruin by the overexploitation of nature, the population being too much for its resource at a time they were no longer in touch with the outside).
The lever. The arm of lever is the length of the handle which is used to multiply by ten the force that you exert ; For example, with an adjustable open-end wrench, multi-grip .. the longer they are, the more the exerted effort is powerful. In the other hand, side displacement of the hand becomes increasingly large. The claw allows to exert considerable efforts because it is a bent bar with a big length on a side of the bent and very short lenght on the opposite side : if the large side is 10 times longer than the small side, the force at the end of the small side will be 10 times stronger ; by exerting a 20 kilos effort, one produces 200 kilos, but it is necessary to have the room because the displacement of the large stem is also 10 times wide ! nothing resists at the claw, nor door alas !! Archimedes would have said ; "give me a lever and I will raise the Earth" If he had one, do not forget the importance of displacement : he would have had to walk a huge distance with this giant lever !
The "launchers of assagai", appeared 20 thousand years ago, increasing the arm of lever and thus the speed of the launching, which could reach 50 metres /second (for a 2 meters assagai). More astonishing, these launchers were sometimes finely carved and decorated, proof of esthetic preoccupations, which draws its inspiration from the need to be distinguished. meter metre
The wedge is even more frightening than the lever ; a good wedge and a good mass handled by a vigorous man make burst anything, even an armoured door. But we have to put it in the
right place, to have enough room hammering it. The principle is that of the lever, the reduction of the effort (when it move forwards
thickness increase); this effect couldn't be to much but the effort of hammering is very powerful.
The effect of wedge based on the expansion/swelling is known since antiquity : In Egypt, the blocks of stone were precut with a line dug in the stone, along which holes were drilled ; then pieces of wood were introduced with force and then moisted ; the swelling of wood, exerting a powerful force, made split the stone. The raw pieces of Egyptian obelisks were extracted thus (an obelisk is cut in solid piece of stone, one says it is a "monolith" (lith=stone).
The screw ; more technical, it makes it possible to also exert considerable power because reduction is significant between the distance around the screw thread (course along the diameter of the screw) and the displacement along the screw itself between two consecutive threads. To the diameter of the screw is at least added the thickness of the nut, or more the arm of lever of the key who claws the sides of the nut. One calls that a screw jack. The jack associated with the parallelogram is used to raise a car ; let us quote the stays of mason who support formwork, ceilings or framings. And at home ? when levelling your washing machine or pieces of furniture and apparatuses, you screw jacks.
The pulley, the belt and the hoist ; the hoist, very old, derives from the pulley of well ; a rope bond at the ceiling goes through a free pulley provided with a hook and goes again through another pulley screwed at the ceiling and finally drop on the floor. one obtains a mere hoist which reduces the effort by two. Usage of more pulleys, will encrease the reduction and as the same time the length of the rope.
If one has two pulleys of different diameter linked by a belt one also reduces (or increase) the effort "at the crank"; this system was used for a long time in mills and industry. With chain and gears, this principle applies to the bicycles, but as the "arm of lever" of the pedals is not enough significant to go quickly, one have to increase both speed and effort, and not to reduce them) ; the first and very slow bicycles were "in direct catch", the pedals fixed on the wheel (a giant one, to compensate).
The reduction by several gears is a succession of simple reductions, whose reports/ratios of reduction are added (wrist clock, clock, gear speed). Digits are multiplying : if the first ratio is 2 (diametre double for the second wheel) and the second ratio is 3 (third wheel triple than the second), the final result 6. With three weels, the final spinning direction is the same than the first wheel.
The hydraulic jack calls upon a technology even more advanced : the pressure of a fluid, in fact the oil, associated with a reduction of the effort based on the oil tranfert between two pump housings : a large pump housing and small one. If we consider the principle "small force with great displacement = great force with small displacement", on the small pump side one needs a great displacement of the pump, whereas on the large pump side (the jack itself), one get a small powerfull displacement ; The hydraulic jack for car is explicit : one gives great blows of pump with a long handle and the car goes up very slowly under the effect of the cylinder, larger, towards which oil is transferred. Please, keen to know the Eiffel tower secret ? the four legs were sustained by the equivalent of big hydraulic jack in order to compensate a possible tilting due to rhe vicinity of the river.
All of us know what it is, thanks to the cars ! the nature, by its "celestial" mechanical organization subjects ourselves to his most direct effect, the acceleration due to gravity (related to the importance of the mass of the bodies).
It is measured by the
We do not feel anything ; it is that the atmosphere of the earth turns with us and that in addition, when the speed is constant, nothing is felt if nothing moves in the vicinity. In the plane, one lamentably trails once in the air, and when one looks at a plane passing, one wonders how it can hold in the air while moving so slowly. All is thus relative. Didn't you ever feel a sort of faintness, in your car or on the train, when the side vehicle starts and that you believe it is you ? this is due to the fact that your body doesn't perceive anything while your eyes believe you are starting. At 600.000 km/h beside something which goes at the same speed, you are quite simply motionless ! I told you, everything is relative.
2/ During the fall of an object, this speed appears independent of its mass (related to the weight) (*). Only the air slows down it more or less ; in a vacuum tube, a feather and ball of steel released together arrive together. You can carry out the test yourself with a light body (piece of wood) and a bolt and you will see that there is little variation (do not throw them from your balcony ! ).
Do not take a feather or a paper sheet, too much sensitive to the air.
3 / In the air, a man which falls freely from a plane reached a speed limit from approximately 250 to 300 km/heure because of the resistance of the air. It can reduce this speed by stretching aside both arms and legs and while keeping an horizontal position "flat on the air", just as wearing baggy clothing. It will hardly go down below 200 km/hour.
formulas : |
Here the theoretical speed reached in the vacuum by our unhappy man falling from 3000 meters height :
Summary: In a vacuum space and under the simple force of gravity, all obects/bodies, whatever their weight and form, reach the same final speed at the end of a given time. Bur air or water or any fluid else slow thoses are larger for the same weight.
3/ Generalization; The constant force which is exerted to an object to put it moving causes same theoretical acceleration as above into 1/ without slowing due to the air, or frictions of the wheels which increases with speed, the speed increases to a same value each second. The mass plays the same role "to obstruct acceleration" but as the force of traction (example of a car), depends on the engine and not on the terrestrial attraction, and consequently is not related to the mass, the value of the mass thus intervenes independently in the formula, just like the force which draws or pushes the car.
Whereas in the case of the free fall, the force which draws to the bas, the gravity, remains proportional to the mass, which makes believe that the mass does not plays any role.
(*) It is also true against a much more powerful car, but only
at the starting and along a few meters, because it is more difficult for the large car to make run its power.
Force (balance. (basis of support) : statement : |
Action, reaction : if you apply a force, it's the action (push, drag, lever, support, roll..). If something resist to the action, it's reaction : example, put a book on the table, the book make an action vertically with its weight and the table react with exactly the opposite force ; yes it is a bit strange, but if the table would not react, the book would go through the table isn't ?
Reaction ; nature invented the jet propulsion : the living organisms among the oldest practise it : jellyfishes, certain bivalvular (shell Saint Jacques..) ; they blow water and move in opposite direction ! B>Jets : the fuel flaring produces gases which are ejected at very high speed because the pressure is very strong ; for that, they must "push" themselves against the combustion chamber of the engine which thus pushes in direction opposed to the open side.
Basis of support : The four legs of a table determine a geometrical area called the basis of support ; if you tilt the table on two of its legs, this area dwindle until the centre of gravity of the table trespass the area which is at this instant a mere line between the two legs : at this moment the table fall over. Your own basis is determined by your two feet ; FUNNY : lean on the wall putting your right side against it (shoulder, hip, leg and foot) try to raise your external leg. You can't because it is impossible due to your centre of gravity which is out of the your basis (the new basis is now only your right foot). Practical experience and safety : in the mountain, going through a narrow path lined both with a cliff on a side and a stiff slope on the other side, be careful because you could be in the same position than "the wall" above, but much more less funny!
Races of formula 1 and damage at the pit for some overheated engines ; could this break down arrive to ? :advice : let still run a little your engineafter the stop to give time at the heat to be evacuated, because the system is not any more in balance and the temperature inside the heated mass goes up abruptly whereas he engine is cut !
Dilation : the hot water which runs less and less quickly at the tap (also, bridges, railways) : it is the dilation of the threaded part carrying the joint which turns off the tap gradually. Solution, buy a new tap with ceramic joints. The proof : cold water does not act. Bridges ; All bridges have expansion joints and they are assembled with kneecaps or rollers at each end because they dilate or contract (see above, the tap). Trains : the "tagada, tagada" noise of the carriages was caused by the space left between the rails for dilation because the problem is serious considering the length ! if we can heard no noise anymore, it is because the joints, numbering much more fewer with the welding of the rails over big length, are produced in long bevels so that one passes from one rail to the other without rupture of rolling. Moreover, one tries to reinforce the contact of the rail with the ground to avoid the excessive heating (heat is evacuated partially).
An iron beam between two walls will make them crack if they cannot move freely !
These molecular forces are considerable and break all, like the dilation of the water transformed into ice, although it is about a phenomenon without any link to the matter (and due,
it is about cold).
The concept of heat or cold is not apprehended by us as the physicists do because the absolute cold is of - 273 degrees for us (about liquid nitrogen). Above, it is heat !! heat comes from a radiation, but the matters take a different temperature under its effect (a metal burns the hand at the strong sun whereas the marble remains fresh).
One succeeded in approaching zero degrees with the following experiment: "multi-stage" boxes are places each inside another and buried in the ground which is at average temperature (18°). Each box, from the smallest to the largest, is painted outside in white and black inside. A probe makes it possible to note the temperature: at the end of a few times, the temperature in the center, in the smallest box, is around 5 degrees. Explanation: the calorific radiation passes easily "through the black", box to box from the smallest and leaves, whereas it is braked with the white of each box in the opposite direction ; heat (radiation) goes out and does not enter.
We are sometimes cold for identical reasons: in front of a pane window, the evening or in rainy weather, we are cold whereas the temperature of the room did not change: we "lose also radiation".
Cold ; refrigerator and bicycle pump. Champagne !
The refrigerator cools the room when it is opened : NO !!, it heats it because its radiator behind provides more heat than the refrigerator make cold (loss of output); the proof ? air-conditioning functions exactly like a refrigerator, but it is necessary to reject heat produced outside, through the windows with a tube or through an external radiator, because you are "in the refrigerator" (it is what the refrigerator does, it rejects heat outside its enclosure). Conclusion, the back of the refrigerator must be well aerated so that the output is correct (and de-iced well).
The bicycle pump acts in the same way : if you pump vigorously, you burn yourselves the hand at the bas of the pump, where the air is most strongly compressed, but when the air escapes, it produces cold around.
Champagne ! When opening a bottle of Champagne, a light and white fume is sometimes formed above the neck : it is not champagne gas but the water vapour contained in the external air which condense above the neck, due to the cold produced by the quick expansion of the gas contained in the bottle when the cork pop up.
Cooling the coffee while blowing air at 37° C ! blowing on the coffee could not be very effective with air at 37° C alone ; in fact, one drives out the vapour above the cup, thus accelerating the evaporation of the coffee ; evaporation produce cold like gas relaxation (compressor of refrigerator or bicycle pump) thus the coffee cools more quickly. this phenomenon is perceptible under the shower when one leaves the cabin, open the curtain or if somebody opens the door ; also after a bath (sea or bath-tub) because evaporation freezes you.
In the very hot countries overseas, one froze in our track literally while leaving the sea by 35 or 40° C outside so much evaporation is strong. Advice : put your bath towels quickly !! |
Making fresh water in scorching sun. yes, its possible : wrap a bottle of water into a floor cloth, place at the sun and wet regularly; one needs patience and a heat wave to have a little fresh water (by the process, you have made a refrigerator "with evaporation" ; sun provide energy.
- When one moves one is cold : when one moves, one loses "our bag of hot air" heated by the body which has settled around oneself (if any draught) and one feel suddenly cold.
- Diving suit : the diving suit does not prevent sea water from entering but it prevents from water circulating and bringing new fresh one against the body ; the plunger thus has a hot water envelope which follows him. It is this hot water which is insulated from the fresh one of the sea, and not directly the body.
- The lid on the pan : itis the opposite effect of the coffee (see above) because in this case it is necessary to prevent the evaporation, which cools, to save energy and heat more quickly.
Dolphins, fish shoal and migration of the birds : it is the over pressure of the water which makes "play" the dolphins in front of the boat : this one creates a wave of over pressure in front of and around the stem on which the dolphins "make surfing".
The water flush and the high- pressure cleaner the flushing are now much less powerful than those of the old WC, with their elevated big tank. To save water, one needs a system which limits the volume of water but not its height in the tank; put for example one or two bricks if that holds without obstructing the mechanism, or suspend a large plastic pocket.. full of water, of sand.. (it must not float!). Do not reduce too much the volume of water (1/4 or 1/3 max because flush would miss energy. There are water savers with two buttons on the market; check their principle of operation. For better including/understanding this concept of pressure and energy, let us continue our reasoning: a simple pipe full of water, high up to the ceiling, brings more pressure than the traditional water flush leaned with the seat, in spite of the low volume of water contained in the pipe. N the other hand, the quantity of water being weak (mass), total energy would be weak and insufficient. Here still, one finds this basic concept of energy, which can carry out "a work", and of power (pressure) , which represents speed with which it is carried out. Conclusion? a bath-tub full of water will not clean the basin properly for lack of power, whereas an elevated pipe starts indeed to clean but ceases fault of "reserve quickly" (of mass to be run out, therefore of total energy). Both are very often needed, but not always. The high- pressure cleaner often indicates 100 or 120 bars and you think that they all are similar with equal pressure; in addition to the quality of manufacture (the pistons and cylinders suffer), the quantity of projected water is a significant element, as for the water flush. With a low flow, you will clean on a small surface, therefore very slowly and incompletely; if you disperse projection, you lose in power. One joined the problem of the water flush; to compare the prices, look at the pressure and the flow. More generally, the pressure stems from the height of water, air, a fluid in general, although any matter is concerned by this principle : example, while drilling a tunnel or in a mine, very dangerous "explosions" of rocks can appear, compressed by their own weight under the mountain. That is also true for man's constructions, traditional or ancient, which rest on load-bearing walls : the latter undergo an enormous pressure at the base; The ancient ones thus built according to a pyramidal form: Egypt, Tower of Babylon, Indian pre-Colombians, just as American ones for their first buildings which have a very broad base and are decreasing. Now, in fact very strong pylons support the plates of the stages, as well as the walls, relatively light or out of glass, which cling literally on the plates.
The high ones with very high water pressures (of liquids in general) are used to cut out, for example rock on several meters thickness, but also to vaccinate; one sees sometimes these kinds of guns which avoid the risks of contamination (jet with strong pressure with a very small quantity injected).
The pressure, which measures a force per square cm, increases with the height and not with the surface. This is why a dam feeding electric turbines must be high; for the same reason, the tide of relatively low amplitude, is not easily usable. A diver sees the pressure doubled at 10 meters of depth, that is to say 2 kg per square cm, because the normal atmospheric pressure value is approximately one kilograms and water depth adds to it 1 kg each 10 meters of water (at 10 meters deep, its lungs narrow and it does not go up automatically).
From a domestic point of view, the flushing are now much less powerful than those of the old ones, with their elevated tank. One can learn a practical lesson from it: to save
>one needs a system which limits the volume of water but not its height in the tank; for example, put one or two bricks if that holds without obstructing the
mechanism, or suspend a large plastic pocket.. full of water, of sand.. (it muss not keep floating !).
There are water savers with two buttons on the market; check their
A better understanding, here are some examples more : a simple pipe full of water, reaching up the ceiling, brings more pressure than the traditional flushing at the back of the seat. In the other hand, the quantity of water being weak (mass), total energy would be weak too and insufficient. Here again, one encounter this basic concept of energy, which can carry out "a work", and this of power, which represents the speed with which it is carried out. Conclusion ? a bath-tub full of water will not clean the basin properly for lack of power, whereas an elevated pipe starts indeed to clean it properly but ceases too quickly by lack of "reserve" (of mass to be run out, that is to say, total energy).
brique>Depression. fish shoal, flights of birds, cyclists, Formula 1..and hoover vacuum.
It is the depression of the air which get birds to arrange themselves in a V shape, and it is sometimes the same reason for the fish shoal.. just as for the cyclists racing. Those of the front line, which are "opening the air", help those of the back line, whose will replace the first soon. For the birds, this depression behind the precedent would save approximately 1/4 of the energy spent by individual. In a Formula 1 race, the pilot "sticks" to the front car to save fuel or to overtake while benefiting from the relative vacuum.
Practical application : a vacuum cleaner of 1500 Watts has a power of aspiration of 30 kPa: 30 kpa means 30 kilo-Pa either 30.000 Pascals, equalizes about to 30.000 divided by 9,81 = 3050 kg per square meter, or 305 grams per square centimetre, or still the equivalent of a water column of 3,05 meters or 3 divided per 13.6 (density of mercury) = 225 millimetres (mm) of mercury (Hg) or also 300 millibars (that's all !! ). It is this aspiration (depression) that it is necessary to compare. To finish, that returns to almost 1/3 of the atmospheric pressure, which is worth in general on the sea level 760 mm of mercury or 1013 millibars (thousandth of bar). With all these equivalences, no manufacturer can more mislead you.
(*) this property of the ice has a serious incidence for the mountain guides when they to drag up a person fallen into an ice crack, because the cord which is attached outside, at a few meters of the edge, inserts in the ice under the effect of the weight (pressure), which is significant, and when the guide arrives near the top, it cannot reach any more the top edge of the crack since the cord is inserted now, below the edge. Arrived close the top, It is necessary thus that it digs a tunnel in the ice to follow the cord and to arrive at surface.
The atmospheric pressure does not vary regularly as the rising weight of the air column varies due to its compressibility. It is denser toward the bottom it should be because it is quite compressed by the weight of air above. It will be even more if it is cold as it has been said above. At high altitude, the air is colder (down to - 56 degrees) and therefore more dense than when it was hot, but as there is little weight on top and it is less compressed, its pressure is very weak. By air, the air temperature must be known to make the necessary corrections to calculate the altitude and the actual speed.
In aviation, they defined by convention what is called a standard atmosphere: the barometric pressure at sea level is equal to
At the bottom of the sea, ie at an average 3,500 metres, the weight of the water exerts a pressure of 3.500 kilogram (3,5 tonnes) per centimetre square : on the bottom, on everything, top, sides, underneath. At ten meters deep, with a little lead around the waist, my lungs reduced in volume by the pressure make me floating in near balance, so with no perception of up and down other than by the sight.
The water is very slightly compressible but its temperature makes significantly varyings its density. water of different temperature or salinity do not mix, cold waters stay at the bottom and warms ones at the top. The Gulf Stream (gulf here means this of Mexico) is a hot surface stream that softens coasts of Brittany, England, Scotland and Norway, and which, too much cooled nearby the Greenland, dives there in two places; it will reappear at the surface, after a nice trip deeper in the waters, thank to the heater in the Gulf of Mexico.
The Mistral cool or cold, doesn't cool the Mediterranean Sea by blowing on it, although he still blows the wapors and participates in this point ; in reality, he drives away the hot waters that are immediately replaced by cooler waters. Three to five days after his disappearance, the warm waters get back and the sea is warm again. Such a temperature change would be impossible in this time given the inertia of the huge mass of water.
In weather forecast, front cold air denser (heavier), slip like a wedge underneath the air of the warm front, lighter.
Just for the physical fussy reality : the pressure at the bottom of the sea is actually the sum of the water and atmosphere pressure, i.e 3,500 + 1 kg/cm2
|Airplanes are pressurized because the air pressure drop when elevating ; at nearly 6000 meters, the pilot of an aircraft not equipped with little
sleep and unconsciousness .. As early as 2000 meters in the mountains, the breath becomes short by lack of oxygen, and the engines losing
power. It is now the Himalayas without oxygen, but with training and therefore high risk.
Those who have been to the Machu Picchu, Peru recall their weakness.
The Laplace's law allows to calculate the air pressure according to the altitude, because we have just seen that the variation along the vertical is not linear .. At 5500 meters, air pressure is 50% lower than the level of the sea. At 10,000 meters, she is only 25% then 5% at 20,000 meters.
In lower altitudes, rather near the sea level, pressure losts 1 hPa (hecto Pascal), or 1 millibar, each 8.54 meters (28 feet / feet) ; lost 10 at 2000 meters. From about 11,000 meters (36,000 feet / feet), it decreases by 1 hPa for 30.50 meters (100 feet / feet).
An altimeter is a barometer graduated in metres. Depending of the pressure, it must be set at the present sea level pressure, or that of the starting level, to be right.
Pressure and depression tend to balance, creating a moving of matter. In weather, the cold front, dense, exert a pressure and pushes lifting under, like a wedge, the warm front lighter. In summer on the coast, the evening breeze, from the earth, which makes shivering when the earth cools while the sea, with its mass and thus its inertia, keep the same temperature. The day, it's the reverse (cool sea breeze) because the land can be much hotter than the sea under the sun, beautiful sea. The wind is generally cool as coming from a colder zone. But there are also hot winds. Without any pressure difference, nothing moves.
Toothpaste dispenser, syringe pump. The atmospheric pressure allows the toothpaste to get up inside the dispenser while pressing the nozzle to get a dose. If the syringe is different in the sense that if the content is slightly viscous (penicillin mixture of glue and water) and the needle very fine, we must pull very strongly because the depression - even very high - fails to completely fill the syringe barrel (see drawing). Why ? At the contrary to the dispenser, the bottom sliding surface provides a significant surface to the atmospheric pressure, the eye of the needle does not receive such assistance, especially as it is added a significant resistance due to the viscosity of the fluid. A heavy needle increase significantly the filling.
Operation of the dispenser of toothpaste (click on image to see much better (three phases are represented by two drawings). The dispenser has two body H (up) and B (down), separated by holes with valves C1 and C2. Pressing the nose and its piston, push the paste contained in H, the valve C2 closes while C1 opens (Figure 2). The dough pressed get out by C1. The height H then determines the dose. In releasing the thumb, the beak and its piston get up under the influence of a spring, creating a vacuum (suction) in H as the valve C1 closes : C2 opens under vacuum and the body B fills the body H : Indeed, the pressure of the atmosphere (1kg/cm square) pushing the sliding lid op going from position 1 to 3 (left diagram, a new position po 3 phase 3).
At the end of the content, we can push the lid to the bottom with the brush head to gain some bonuses doses. A simpler system exist, but less precise, with an upper deformable instead of a beak with piston.
Defusing (toothpaste tube, suction pump). Defusing occurs when an air pocket grows in the pump body, usually by a leak in a gasket or piston. The air cut "then the continuity of the pumped liquid or paste (toothpaste, water, gas ..) and the pump will not pump anymore, temporarily or permanently, as it "suck inefficient air bubble. It must then "reboot" by evacuating the air. For the toothpaste is simple: you push the piston with the head of the toothbrush, which drives out air bubbles and makes them out.