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Difficulty understanding newtonian mechanics with stationary force

  1. Nov 24, 2012 #1
    I apologize if this seems much too "freshman", and may come across as annoyingly simple to some more trained or educated on this board. However I will ask the question anyways as I have failed to find an answer elsewhere.


    Under Newton's Second Law of Motion, f=ma.
    thus F = kg (m/s^2)

    But when a person pushes against a wall like in the picture below, there is no acceleration, a=0.

    Fphoto_61390509_C_3_BK.jpg
    The person and the wall have 0 velocity and 0 acceleration.

    If a=0 then F=m(0), F=0
    How can there be no force here (treating it mathematically according to the formula), when the person is exerting a force upon a body(the wall)?


    ---

    And secondly, if an object of a certain mass and velocity, is in motion but not accelerating,
    Newtons second law also would calculate that there is no force according to the formula.

    Say a 4 kg bicycle moving at a consistent velocity of 2 m/s, that has 0 acceleration.
    http://www.theglobalintelligencer.com/images/bicycle.jpg

    F=(4kg)(0) = 0
    I know the momentum p, would equal p=(4kg)(2m/s) from p=mv.

    But there is no momentum without a force being exerted upon the object. Force is coming from the rotation of the tire via the humans legs rotating a gear (translating an general up and down "force" into a rotational "force", giving the bicycle (the object) a momentum.
     
  2. jcsd
  3. Nov 24, 2012 #2
    This is from the Wikipedia article on Newton's laws of motion
    http://en.wikipedia.org/wiki/Newton's_laws_of_motion

    See if you can apply the correct law(s) to the person pushing on the wall, and to the bicycle.
     
  4. Nov 24, 2012 #3

    rcgldr

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    That's only true if there are other forces that result in the net force on the person and the wall summing up to zero. For a simple case that only considers forces originating from the person, the person pushes backwards on the surface of the earth, and forwards on the wall. The surface of the earth pushes (or pulls) backwards on the wall, and the net sum of forces on the person and the wall is zero.
     
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