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Bend the knees

  1. Feb 18, 2009 #1
    If a person is standing on a chair and jumps off, does the floor feel the same force whether the person bends their knees or not?

    I suspect the floor feels the same force regardless, as the energy felt by the body is absorbed by the muscles, but the floor feels the same equal and opposite force. I see this as Newtons's 2nd law. thanks!
  2. jcsd
  3. Feb 18, 2009 #2


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    Then there wouldn't be much point in bending your knees!

    The force on floor depends on your mass and how quickly you stop F = ma
    By bending your knees your legs act like a spring and allow you to stop more gradually - since the deceleration is less, the force on the floor and so on you is less
  4. Feb 18, 2009 #3
    The floor would feel the same total force regardless of weather you bent your knees or not as you land on the floor. The key factor here is time. If you bend your knees when you land there is less force felt on the floor for a longer time. If you don't bend your knees there is a larger force felt on the floor for a very short time. Albeit some might argue that the resulting pain from such an endeavor would be felt for quite some time afterwards.
  5. Feb 18, 2009 #4


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    The total energy into the floor is the same - but the peak force is less.
  6. Feb 18, 2009 #5
    What on earth do you mean by total force? Perhaps impulse or maybe energy are the words you're looking for?

    And the force would definitely be reduced. The initial momentum and energy would be the same, but the force would act over a longer amount of time thus the force would be reduced.
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