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Which Moves 1st?

  1. Oct 2, 2004 #1
    My son asked me a question....


    A block is placed on a wedge... if there is no friction on any surface, which will move 1st?

    any thoughts?

    Rick :confused:
     
  2. jcsd
  3. Oct 2, 2004 #2

    Tide

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    They will begin to move simultaneously since each is exerting a force on the other simultaneous with the other.
     
  4. Oct 3, 2004 #3
    I agree, they will move simultaneously.
     
  5. Oct 3, 2004 #4

    HallsofIvy

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    In fact, since there is no net horizontal force, the center of mass MUST remain on the same vertical line. If one moved before the other that could not be true.
     
  6. Oct 4, 2004 #5
    what moves first

    O.K. They move together as an approximation.

    But, let as see what actually happens. First, gravitational force acts on the block. Simultaneously the motion of block starts. Now, the block exerts a normal force on the wedge. The force is transmitted in a finite time. So, after that finite time, the wedge starts moving. That finite time is too small.

    But it still is there.

    What do you say?

    spacetime
    www.geocities.com/physics_all/index.html
     
  7. Oct 4, 2004 #6
    The block and wedge are in direct contact with each other, which means that the force would be "transmitted" instantaneously. However, the forces are really not separable from one another: the wedge exerts normal force on the block - and the block exerts weight on the wedge - as soon as they come into contact. There is no delay whatsoever.
     
  8. Oct 5, 2004 #7

    HallsofIvy

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    No, that is not an approximation. Unless you are going to violate conservation of momentum, any move by one block MUST correspond exactly by a move by the other.
     
  9. Oct 5, 2004 #8
    I think you will make my work easy if you could please tell me, that if, you are standing in front of mirror and switch on a bulb at you back from where light will you see earllier i mean reflected one or one by bulb itself.
    If you could answer that i think you know the solution yourself.
     
  10. Oct 5, 2004 #9

    Tide

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    Unfortunately, you don't get to turn gravity on or off with a switch. It was there and "turned on" when you first placed the block on the wedge. Newtonian physics simply does not account for propagation of gravity.
     
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