How do hydraulics not "create energy"?

I was using a car jack today and I put very little effort into pumping the handle and it raised a 3000lb car.

I get the premise of how hydraulics work. I have looked at some diagrams. I am just thinking intuitively it makes no sense. to me that it actually works. I put probably 20 lbs of force on the jack and it somehow lifts a 3000lb car.

Comments

  • http://hyperphysics.phy-astr.gsu.edu/hbase/pasc.ht...

    If you go to the website above, you will see a drawing that shows how a hydraulic jack works. The hydraulic jack is composed of two cylinders that are connected by a short piece of pipe. Each cylinder has a piston inside of it. The cross sectional area of the cylinder that is supporting the car is much larger than cross sectional area the piston that is attached to the handle of the jack. The pressure is the same in both cylinders.

    Pressure = Force ÷ Area

    As the area decreases, the force decreases. The main difference is the distance each piston moves. If you look at last drawing, you will see the small piston has moved a greater distance than the large piston. As the small piston is pushed downward, a specific volume of hydraulic fluid moves from the small cylinder to the large cylinder. As this happens the car moves a small distance upward. If we assume that there is no friction, the work that is done by the small piston is equal the work that is done on the car.

    Work = Force * distance

    As the distance increases, the force decreases. The ratio of the distances is equal to inverse ratio of forces.

    3000 ÷ 20 = 150

    If you go to the website below, you can read more about how Pascal’s equation is related to a hydraulic jack.

    https://www.grc.nasa.gov/www/k-12/WindTunnel/Activ...

  • Energy in the English system is measured in "foot pounds." The hydraulic jack just a way to convert the foot pounds you put in at the handle into raising the car by a few feet.

    For raising the 3000 pound car by two feet, it takes 6000 foot pounds of energy. You have to apply 25 pounds of force on a four foot handle arc 60 times to equal that.

    2 feet X 3000 pounds X 1 time = 4 feet 25 pounds X 60 times.

    It may take a few more times to to fill the channels in the jack with hydraulic fluid, but that's the principle.

    One cycle of the jack, 4 feet times 25 pounds raises the 3000 pound car by by 12/30 inches or 0.4 inches.

    You are trading a small force over a large distance for a large force for a small one. The energy and work is the same.

    http://www.physicsclassroom.com/class/energy

    This is one of the few problems easier to do in the English system. The metric system involves kilograms, meters, joules and newtons. The last two are not very familiar even to users of the metric system.

    For reference, a bottle of beer has about 1000 kilojoules of energy and 300 nutritional calories. That's not much energy, but if you drink enough of it, the effect builds up.

    I am a mechanical engineer with extensive education in physics.

  • Acer makes a very valid point.

    Your application of 20 lbs is applied over a distance that is considerably longer than the height to which you lift your 3000 lb car. Therefore you are doing the same amount of work (actually more because some of your effort is lost as heat).

    The physics basis behind a hydraulic system is that the pressure of the hydraulic fluid is the same throughout an enclosed vessel. The handle applied this pressure over a much smaller area that the piston that lifts the car. The same pressure applied over different areas results in the force multiplication.

    F = PA

    The tradeoff is the distance moved.

  • Force is not conserved, energy is what is conserved. In terms of mechanical work, energy is force times distance. The jack allows you to input a huge amount of energy into the car using a small force by applying the force over a long distance. The car only went up a foot or so, but you pumped in hundreds of feet of hydraulic fluid (as measured at the point you are pumping). All basic machines (gears, levers, ramps) work this way: increasing the distance over which a force is inputted to allow a smaller force to be used and still arrive at the same amount of energy inputted.

  • A car jack exploits Pascal's principle - that is to say an enclosed fluid under pressure in a pair of cylinder rams (one small and one large) exerts that pressure throughout its volume and against any surface containing it. This means the larger cylinder can exert a far greater pressure as a result of the appliction of a relatively small amount of force in the smaller cylinder. What it can't do is create energy - the trade off for the greater force is that the distance moved in the rams is inversely proportional to the amount of pressure applied - but this is a small price to pay for the result.

  • The car moved up maybe 1 foot and therefore gained gravitational potential energy. You did several (30?) strokes on the handle and each stroke might have been 1 foot.

    If you calculated the car's gravitational energy and the work you did with all your strokes, you would find that you did more work than the energy the car gained (friction and heat being the reason it wasn't equal).

  • One of things working for you is the one way valve. As you pump fluid into the lifting cylinder, the one way valve prevents back flow, which would stop the cumulative effect of repeated pump action. Also, if you put 20# of force on the jack handle, the length of the jack handle multiplies this force. If the handle is 20 inches long, the force is 20*20 or 400 pounds into the pump.

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