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Will this force be zero ?

  1. Feb 6, 2016 #1
    1. The problem statement, all variables and given/known data
    I am pushing a very heavy ball with full strength, but the ball is not moving at all. So, there is no change in momentum of the ball.
    Is it appropriate to say that the Force applied to the ball is zero ?

    2. Relevant equations


    3. The attempt at a solution
     
  2. jcsd
  3. Feb 6, 2016 #2
    What are your thoughts on this?
     
  4. Feb 6, 2016 #3
    Force is rate of change of momentum. But here change of momentum is zero, so Force should be zero.
     
  5. Feb 6, 2016 #4

    SteamKing

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    Look at it another way. While pushing on the ball, you slip and the ball rolls on top of you and stops. The ball is not moving, yet it is crushing you. Is the force still zero?
     
  6. Feb 6, 2016 #5
    Change the ball to say some other rigid body which is incompressible anyhow, with very heavy mass. I am pushing it but can't make it move. Then ?
     
  7. Feb 6, 2016 #6

    SteamKing

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    There's a building which you are pushing against. The building does not move, yet your muscles are straining to push it over. Are you exerting no force against the side of this building?
     
  8. Feb 6, 2016 #7
    What is the difference between force and net force?
     
  9. Feb 6, 2016 #8
    Of course I am exerting pressure. But in physics terms (Newton's 2nd Law), Force should be zero.
     
  10. Feb 6, 2016 #9

    SteamKing

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    Since pressure is defined as force per unit area, if you are exerting pressure, ipso fatso, you are exerting a force. :wink:
     
  11. Feb 6, 2016 #10
    Newton's 2nd law says that the NET force acting on a body is equal to its mass times acceleration. It doesn't say that any individual force on a body is equal to its mass times acceleration.

    If you are pushing on a building (and the building isn't moving), what are the two horizontal forces acting on the building?
     
  12. Feb 6, 2016 #11

    Nidum

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  13. Feb 6, 2016 #12
    If you are pushing a rigid body and the body doesn't move
    It's only so because the "net" force acting on the body equals zero yielding zero acceleration
    ΣFexternal = m×a
    Where m is the mass and a is the acceleration produced in the body
    This is newton's 2nd Law of motion (simplified)
    Look at it in this way
    Let's say you're pushing a block on a smooth table and your friend is pushing the block in the opposite direction
    Let's say both of you keep on pushing with equal forces but the block doesn't accelerate
    Does this mean that you aren't applying a force on the ball?
    You are right as you're pushing the ball
    However the net force "applied"to the ball is zero as the forces of equal magnitude in opposite directions cancel each other out
    Hence it's net force, not an individual force acting on a body which yields the acceleration!!




    UchihaClan13
     
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