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I'm having a bit of trouble figuring out which direction kinetic friction is supposed to act in.


A glass of water is placed on a cloth at distance d from the table edge, and the cloth is spread out across the table until length l from the same table edge. The cloth is pulled at constant velocity v towards the edge of the table, and is pulled at a speed such that the l-d length of cloth slides under glass and the glass remains under the table. Would the friction between the glass and the cloth act in the direction the cloth is being pulled or in the other direction?

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I think that the kinetic friction would act in the direction the cloth is being pulled. Think it this way like your are on top of the cloth and moving with acceleration 'a' of the cloth then glass will appear to be moving on opposite direction and as frictional force opposes relative motion it will act in the direction the cloth is being pulled.

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  • $\begingroup$ So are you saying that because relative to the cloth, the glass moves away from the table edge, so the frictional force acts in the direction towards the table edge. And since the table cloth moves at a constant speed we are in an inertial frame so Newton's laws of motion applies? $\endgroup$
    – BigWig
    May 3, 2018 at 19:15
  • $\begingroup$ Yes, relative to the cloth the glass moves away and so frictional force will oppose that relative motion and act towards the direction the cloth is being pulled. $\endgroup$
    – Vaibhav
    May 4, 2018 at 20:25

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