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Friction force homework problems

  1. Feb 28, 2008 #1
    Hey guys this is a practice problem for my lab and i cant get it can someone please help:
    The force sensor measures the force on the sensor due to the bumper, but the cart's momentum change arises from the force on the cart due to the bumper. Which of the following facts are needed to assert that the magnitude of these two forces are nearly equal at all times.

    A. Their magnitudes differ by the magnitude of the net force on the bumper.

    B. The force of friction is small.

    C. The net force on the bumper is small.

    D. The cart's weight is canceled by the the normal force exerted by the track.



    1. A, B

    2. A, C

    3. A, D

    4. B, C

    5. B, D

    6. C, D

    7. A, B, C, D

    Thanks!
     
  2. jcsd
  3. Feb 28, 2008 #2

    kdv

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    can you describe a bit the physical situation? Where is the sensor, what is happening? It's a colision? The bumper is on what? It's hard to reply without some idea of the setup.
     
  4. Feb 28, 2008 #3
    Well I have not done the lab yet, and these are practice pre-lab questions. But usually the force sensor is attactched to the end of the cart which is on a "frictionless" surface. It is an impulse and momentum lab and i believe that the bumber is on the end of the meterstick.
     
  5. Feb 28, 2008 #4

    kdv

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    Ok.



    Basically, the change of momentum is due to the net force on the cart. While the sensor is measuring only the force on the cart due to the bumper. The question is then: why are these two forces nearly equal?
    Another way of saying this is: why can you ignore all the other forces on the cart, beside the force exerted by the bumper?

    Hint: what are the forces on the cart during the collision? Why can you ignore all the other forces beside the bumper force?
     
  6. Feb 28, 2008 #5
    so then it is
    A. Their magnitudes differ by the magnitude of the net force on the bumper.

    B. The force of friction is small.

    A and B correct?
     
  7. Feb 28, 2008 #6
    actually the answer is all of them isnt it?
     
  8. Feb 28, 2008 #7

    kdv

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    I would not agree... Two of them sounds right to me.
     
  9. Feb 28, 2008 #8
    is it b and c?
     
  10. Feb 28, 2008 #9

    kdv

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    What are the forces acting on the cart during the collision?
     
  11. Feb 28, 2008 #10
    well the surface is considered frictionless but thats impossible right? there has to be some friction and the other is the applied force right?
     
  12. Feb 28, 2008 #11

    kdv

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    Force applied by what? You are also forgetting the forces along y
     
  13. Feb 28, 2008 #12
    o0oh the normal force and the force due to gravity, but since the motion is horizontal i thought those didnt matter...so im guessing the answer is b and d which are friction and normal/gravity
     
  14. Feb 28, 2008 #13

    kdv

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    So during the collison, there is the force due to the bumper, a little bit of friction, the normal and the weight.

    The question is : why can't we forget about the normal, the weight and the friction is say that the net force is basically just due to the force of the bumper?

    there is no reason to say that the force of the bumper is much larger than the weight of the normal but we can forget about those because they cancel out.

    And we can forget about the friction force because it must be much smaller than the bumper's force.


    Choice A says that teh magnitude of the bumper's force and the net force differ by the net force on the bumper. This does not make sense. And there is nothing that say sthat the bumper force is small.

    So here you go...
     
  15. Feb 28, 2008 #14
    ok i think i got it now C and D?
     
  16. Feb 28, 2008 #15

    kdv

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    Why do you have to assume that the force due to the bumper is small? If it was, we could not neglect friction compared to it.
     
  17. Feb 28, 2008 #16
    sorry wrong choices i meant B and D
     
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