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No friction

  1. Nov 4, 2004 #1
    Let say we have a sliding box. The box is sliding because of an constant force perpendicular to the box. There is no friction and no air restistance. Does the speed of the box increase or is it constant. ?
  2. jcsd
  3. Nov 4, 2004 #2
    Newton´s law: F = m a
    If you have a force (sum of all forces in the system, also friction) that is not zero, you always have an acceleration...
  4. Nov 4, 2004 #3

    Doc Al

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    I assume the box is on a horizontal, frictionless surface, and that the force "perpendicular to" the box acts horizontally?

    Assuming the only horizontal force is the one mentioned, then the box will accelerate. Note: I would not say that the box is sliding because of the force, since a force is not required to maintain motion. The box is accelerated because of the force.
  5. Nov 4, 2004 #4


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    Depends on the shape of the box.On normal basis,if the force doesn't push the box to the ground and the the box be motionless before the application of the force,it could accelerate on any direction.
  6. Nov 4, 2004 #5
    It isn't correct to say that it depends on the shape of the box. It will always accelerate, unless the plane is horizontal. It will not move to 'any' direction, either.
  7. Nov 4, 2004 #6
    Thanks, the answer is what I thought. Maybe it would be clearer to imagine the box in space with a constan force acting straight on, then the box would accelerate, thought the force would stay the same, say 100 N.
  8. Nov 7, 2004 #7
    well if the force is perpendicular to the surface, and if we assume the box and the surface are unbreakable, the normal force of the surface would cancel out the constant force, so it cant be sliding at all. Gunnar, please give more information or change the scenario.
  9. Jan 2, 2006 #8
    I'm sorry, I misunderstood. Is the scenario like this?
    .F -------->.||..........||...........

    If so, and if F is the only force acting on the box (besides gravity, but since there is no friction, it is negligable) then the box will be accelerating.
    Last edited: Jan 2, 2006
  10. Jan 2, 2006 #9


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    On the contrary, what Dextercioby said was completely correct (although possibly slightly confusing to the original questioner). Nothing was said about the force going through the center of gravity. Depending on the shape of the box, some or all of the force might simply cause the box to rotate around its center. Since nothing was said about the direction of the force, it could also move in "any" direction.

    I am also puzzled by "unless the plane is horizontal". The question ("The box is sliding because of an constant force perpendicular to the box.") implies no other (gravitational) force so it would be natural to consider that the plane is horizontal- and will certainly accelerate. (Perhaps with only angular acceleration but I doubt the original post intended that.)
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