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This might seem like a stupid question, but my instructor could not give a straightforward answer.

If a body is kept in contact with a completely vertical surface i.e. right angle to the ground, and only force on the body is the force of gravity straight downwards.Then normal force on the body would be zero. We know that frictional force on the body is proportional to the normal force, hence it is also zero. Does that mean there is no friction whatsoever, despite the surface rubbing?

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marked as duplicate by sammy gerbil, ZeroTheHero, M. Enns, Buzz, John Rennie newtonian-mechanics Jan 12 at 9:42

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

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    $\begingroup$ Without normal force, then there is no surface rubbing. If any rubbing happens, because you initially have them squeezed together before letting go, then there is also a normal force. But the object will move away from the surface (due to this normal force) quickly and continue the fall without any further rubbing or normal force or friction. $\endgroup$ – Steeven Jan 11 at 6:57
  • $\begingroup$ @Steeven Why wasn't this posted as an answer? $\endgroup$ – Aaron Stevens Jan 11 at 12:36
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The ideal answer is that there is no friction. The reason why is what you give: since no horizontal force acts on the body, there is no normal force between the body and the vertical surface. Therefore the surface aren't pressed into each other, and so there is no friction force.

Of course this is not obtainable in the real world. Surface irregularities will cause variations in the way the two surfaces come in contact and interact. This could even result in the object losing contact with the vertical surface altogether. The friction model of $f=\mu N$ is a very simplified model, and one instance it breaks down is in scenarios where the force between the two objects is hard to define. This is such a case.

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For a perfectly smooth and idealized wall and body there will be no friction in this scenario. Real-world objects are not perfectly smooth, so there will be interactions as jagged pieces catch on each other, giving rise to a non-zero normal force. This is what you are terming "rubbing".

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