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Homework Help: Impossibility of unforced movement on a frictionless surface

  1. Dec 3, 2015 #1
    1. The problem statement, all variables and given/known data

    New to physics forum, so please forgive me if I am posting this in the wrong place, but it seems to me that this is a homework-type or basic physics question. Here it is: You have a perfect cube with substantial mass sitting on a flat frictionless surface. The surface plane is parallel to the surface of the earth (the surface normal is parallel to the gravitational force). The surface is moved horizontally, remaining at all times parallel to the earth's surface. It seems to me that, irrespective of the speed of movement of the surface, whether very fast or very slow, the cube will never move, as the surface is frictionless. Without friction, how will a horizontal force be applied to the block. The surface will simply move under the cube, which will remain in its resting position in space.

    Obviously the cube is sitting on the surface (touching the surface). But no friction. So how can it move under these ideal conditions? And how can something be touching but no friction? Is this paradoxical behavior in this ideal situation.

    2. Relevant equations

    3. The attempt at a solution

    I've thought about it, to no avail.
  2. jcsd
  3. Dec 3, 2015 #2


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    You've got the answer. The only thing you needed to add was that the idealised situation of a totally frictionless surface is, of course, physically impossible.

    But, in fact, another way to think about the term "frictionless surface" is that it really means "friction is sufficiently small to be ignored".
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