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Momentum questions

  1. Mar 23, 2009 #1
    1. The problem statement, all variables and given/known data
    http://img16.imageshack.us/img16/5889/physics.jpg [Broken]

    3. The attempt at a solution

    Well for question 5, I'm having a bit of trouble recognising the reason why the space-pod doesn't move after they take their seats. I understand that the centre of mass doesn't change but I can't see the reason.

    As for question 6, I'm wondering what bearing the 'same frictional force' has on the situation. When I first looked at it I thought that the kinematics equations could be used but now I'm thinking I have to do something with that 'same' friction force.

    Any help greatly appreciated.
    Last edited by a moderator: May 4, 2017
  2. jcsd
  3. Mar 23, 2009 #2


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    5) When Simma and Stan are swapping, they can do so only by pushing the floor of the space pod. After they take their new seats, there in no action on space-pod by them. So the space-pod does not move.
    Initially all of them are at rest.( Including space pod ). When they are swapping, they are pushing the floor of the space pod in the opposite direction. Assuming that they take the same time to swap the seats, calculate the change in momentum of Simma and Stan, and equate it to the change in momentum of the space pod.
    In problem 6, after pushing each other, they can come to rest due to the friction force.
  4. Mar 23, 2009 #3
    Great thanks a lot rl.bhat, I'll try have a crack at those properly now.

    Oh, I also have another quick conceptual question. Why is it when you are on a rotating surface that it goes from kinetic friction to static friction in the centre? I hope you understand what I mean. I'm saying as you walk and the radius gets smaller why does it become static friction? I'm interested to know the actual physics behind it. Is it something to do with the magnitude of centripetal force or the tangential velocity?
  5. Mar 23, 2009 #4


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    When you keep any object on a rotating surface, its tendency is to fly away radially. If you want to prevent it from sliding, there must be sufficient kinetic friction between surface and the object.
    Last edited: Mar 23, 2009
  6. Mar 23, 2009 #5
    I'll show you the question and why I'm asking such a conceptual question. It's a bit of a weird one. I get how to find the radius of static friction but I don't understand the concept. I know it's best it walks from outside to inside.

    http://img257.imageshack.us/img257/2746/phs2.jpg [Broken]
    Last edited by a moderator: May 4, 2017
  7. Mar 23, 2009 #6


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    when a car is negotiating a curve, the passenger is thrown away radially.
    Similarly the robot will be thrown away radially with a force mv^2/r. To prevent this motion there should be equal amount of kinetic friction toward the center.
  8. Mar 23, 2009 #7
    So when this slipping stops, static friction takes effect? What I'm trying to find out is the reason it slips on the outside and stays in the inside. Thanks for your help, and for being so patient.
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