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Why two men can't lift a 2 ton car but can push same car

  1. Nov 25, 2014 #1
    Hi Guys,

    I am wondering that what physics involved here.

    1. Why two men can't lift 2 ton car at 1 meter height but can push same car up to even 1000 meter.
    2. At start car push takes much energy but later it requires less energy to push

    So weight is same but 2 men can't move car vertically but can move car horizontally up to long distance?

    Also specially once moved, why its easier to keep pushing car.

    What physics and equations involved here. Can any one post here?

    Shaks
     
  2. jcsd
  3. Nov 25, 2014 #2

    Simon Bridge

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    1. What supports the weight of the car when lifting vs when pushing?
    2. Look up "rolling resistance", and "static friction" as opposed to "kinetic friction".

    ... look up Newton's Laws of motion. Momentum keeps the car moving without any force at all.
     
  4. Nov 25, 2014 #3

    russ_watters

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    Consider power vs energy.
     
  5. Nov 25, 2014 #4
    What if the car is suspended in air by a chain, and the chain goes via a pulley so the chain is horizontal after the pulley, and some handles are attached to the end of the chain. Could the two men push the handles such that the car was lifted? What if the car rested normally on the ground and there was no pulley, just the chain?
     
  6. Nov 26, 2014 #5

    Drakkith

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    There is a great amount of downward force exerted on the car by gravity, and 2 men by themselves are incapable of exerting a greater amount of force against gravity (upwards), so the car doesn't move when they try to lift it. In contrast, moving a car up a ramp allows you to exert a smaller amount of force over a larger distance to move the car.

    See here: http://en.wikipedia.org/wiki/Inclined_plane
    And here: http://en.wikipedia.org/wiki/Mechanical_advantage
     
  7. Nov 26, 2014 #6
    Go through Newton's law.
    Consider free body diagram of car.
    When pushing consider free body diagram of wheels. Take static and kinetic friction.
    Static friction acts till you start moving the car. Although friction opposes applied force it supports rotation of wheels.
     
  8. Nov 27, 2014 #7
    This is the easiest answer to understand for a beginner like me. :)

    Thank you everybody.

    Shaks
     
  9. Nov 27, 2014 #8

    Drakkith

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    One question. Were you asking why two men could push a car across 1,000 meters of flat ground, or why they could push a car up 1,000 meters in height even though they can't directly lift it?
     
  10. Nov 27, 2014 #9
    This one

    So the reason they need less energy to keep pushing car on ground is because of momentum.

    What's equation of momentum for linear (this car example) and object moving in vertical circular path?

    Shaks
     
  11. Nov 27, 2014 #10

    Drakkith

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    Ah, my mistake then. I thought you were asking something else. In that case, ignore the part of my answer about the ramp and pay attention to the stuff about friction that was said by others.
     
  12. Nov 28, 2014 #11
    If the man has enough pulleys attached to the roof and to the car. The man can lift the car by himself. I know because I did this! In a sciencemuseum. (Heureka, in vantaa in finland)

    But the question of friction seems good question.

    Lets say you are a homemover who is carrying stuff into the client's new house.
    If you have a heavy movingbox containing 20kg of stuff.

    It should be way easier to push the box along the ground compared to lifting and carrying the box in your arms. Let's imagine that the floor does not suffer damage

    In some african countries women often pick up drinking water from a well outside village. These women balance the water container with their hands upon their head. They suppirt the container with their hands that is.

    I guess i was wondering whether thats a sensible carrying method. Of course wheelbarrow would be more effective.
     
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