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Newton's Second and Third Laws

  1. Oct 16, 2008 #1
    1)
    1. The problem statement, all variables and given/known data
    A force of 1.5 N is applied to a block on a flat surface and the block moves with a constant velocity. Approximately what applied force is needed to keep the block moving with a constant velocity that is twice as large as before?

    2. Relevant equations
    F = ma

    3. The attempt at a solution
    At first I thought the answer would be 3 N, but now I'm not sure. When you originally apply the force of 1.5 N the force of friction must be 1.5 N as well so that the block moves with a constant velocity right? So then if you increase the force to 3 N would friction increase to 3 N so that the block moves with a constant velocity?


    2)
    1. The problem statement, all variables and given/known data
    A 90 kg man and a 60 kg boy each have one hand extended out in front and are pushing on each other. Neither is moving. Eventually the boy begins to slide across the floor. Compare the forces between their hands.

    2. Relevant equations
    None?

    3. The attempt at a solution
    When neither is moving the forces between their hands are equal according to Newton's third law. But once they start moving does this still hold true? My guess is that the forces are still equal but have increased in magnitude so that's what causes the boy's acceleration.
     
  2. jcsd
  3. Oct 16, 2008 #2

    Redbelly98

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    (1)
    Recall the general formula for friction force. How does it change (or does it) when velocity is doubled?

    (2)
    I agree with your answer.

    p.s. Welcome to PF.
     
  4. Oct 17, 2008 #3
    Okay, I remember now that friction force doesn't increase with velocity. In that case, pulling with a force of 3 N would cause acceleration. Then is it even possible to move the block with twice the constant velocity?
     
  5. Oct 17, 2008 #4

    LowlyPion

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    Yes. Any excess of force over the constant motion state of 1.5N will serve to accelerate the block. Once at double the speed how much force is required to maintain it at that speed?
     
  6. Oct 17, 2008 #5
    Oh so does that mean that after the block has accelerated, you only need to apply a force of 1.5 N to keep it moving at twice the constant velocity?
     
  7. Oct 17, 2008 #6

    LowlyPion

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    If there was more it would accelerate no?

    If less it would slow.

    Looks like it to me.
     
  8. Oct 18, 2008 #7
    Thank you!
     
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