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If the surface object B is placed on is frictionless, on applying a force F on object A, friction will act on object A opposite to the direction of the force applied (opposite to the direction of its tendency of slipping relative to B/ actual slipping relative to B) and will act on object B in the direction of the force applied (opposite to the direction of its tendency of slipping relative to A/ actual slipping relative to A.)

The surface B is on being frictionless can only be an ideal situation, but in this case, friction opposes object A’s tendency of motion towards the right while it enables B’s motion to the right. Is friction enabling relative motion between A and B here?

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enter image description here No my friend you are absolutely wrong. You can see as the force $F$ is acting on the upper block it has to first cross the limiting friction barrier so as to induce slipping between the two blocks . As you can see when $ F $ acts on upper block it has tendency of giving a kinetic energy to it and hence the upper block has a tendency to move towards right so friction on the upper block acts towards the left. But as we know the lower block is on a smooth surface and is not being acted by any external force ( to the system of two blocks) hence it appears to move towards the left with respect to upper block and hence friction on it acts towards right so as to impart it some kinetically energy so that the blocks move together with no slipping . But notice :- as mentioned let's assume another case for deeper insight let $ F $ be varying with time as $ F=g(t) $ in this case till the force F doesn't cross limiting friction value the two blocks move with same acceleration but after that the friction becomes kinetic in nature and now their acceleration are no longer the same . enter image description here

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Does friction enable relative motion between masses A and B?

  • To answer this question we must explicitly define the word "enable".
  • Enable - def: to cause to be active or available for use.
  • Does friction make relative motion available between masses in surface contact?
  • The answer is no. Friction does not enable relative motion between A and B in the sense of increasing relative velocity. Friction does the opposite.
  • But, friction does enable motion between B and the table. (External force F accelerates A. Both static and kinetic friction between A and B transmit force to B, which then accelerates in relation to the frictionless table. But, friction does not enable/increase the relative velocity between A and B. The force transmitted to B by friction accelerated B, but the relative velocity was reduced because of friction. This is not "enabling" relative motion, this is reducing/inhibiting relative motion.
  • Friction never "causes" relative motion between two surfaces in contact. This would produce spontaneous acceleration/perpetual motion. Friction never accelerates a mass, rather, friction always decelerates a mass because it converts kinetic energy into heat. Friction does not enable relative motion of masses, it inhibits relative motion of masses in surface/friction contact.
  • It is true that B is moving because of friction between A and B. But, saying friction enabled relative motion between A and B is misleading because friction reduces the maximum relative velocity between A and B if the surface between A and B was frictionless.
  • Friction does the opposite of enabling relative motion between two masses.
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No. The applied force causes a tendency for A to slide rightwards away from B. This is equivalent to B sliding leftwards away underneath A.

Friction counteracts this on both blocks. It pulls leftwards in A and rightwards in B to counteract the effect of the force.

Friction balances it out. It will never cause relative motion but only counteract it.

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