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Homework Help: Frictionless Surface

  1. Oct 23, 2007 #1
    1. The problem statement, all variables and given/known data

    A small cylindrical disk hits a more massive square block directly head on. Both objects slide without friction on a smooth ice surface. The block is pushed forward and the disk bounces back. Which statement is true?

    a) The block pushes the disk.
    b) The disk exerts a greater force because it is moving.
    c) The block exerts a greater force because it is more massive.
    d) The total momentum changes when they collide.
    e) No forces occur because the objects are just sliding (inertia).

    2. Relevant equations

    Newton's 3rd Law and Conservation of Momentum.

    3. The attempt at a solution

    I've narrowed down my answer to either A or E because of Newton's 3rd Law and Conservation of Momentum.
  2. jcsd
  3. Oct 23, 2007 #2


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    What does "the disk bounces back" indicate with respect to forces?
  4. Oct 24, 2007 #3
    I don't get what you mean. All I know is that the objects exert the same forces on one another, right? So that is how I narrowed it down to either A or E. But is it one of the others? Can someone explain?
  5. Oct 24, 2007 #4
    Newton's 3rd law tells us that when you punch the wall, the wall punches back equally. In other words, with equal force. In other words, you don't punch the block with any more force than the wall punches you with. In other words, your punch's force and the wall's punching force are equal.

    So take what I said above, and replace your punch with the cylinder, and the wall with the block. Does one hit the other with greater force? Or are they equal?
  6. Oct 24, 2007 #5
    I understand Newton's 3rd Law. So does that mean that the answer is A? Because there are forces that are occurring, even if it is a frictionless surface. So the answer is A. Correct?
  7. Oct 24, 2007 #6
  8. Oct 24, 2007 #7


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    E is false, because the disk is subject to a force by virtue of the fact that it changed direction and momentum, therefore a force had to be involved.
  9. Oct 24, 2007 #8
    Okay. That makes sense Astronuc, thanks. Thanks to you too dvdqnoc
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