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Lets take for example a mass pushed by some force F. The mass rests on a horizontal surface with friction. F is large enough that the object is moving and a kinetic friction force is applied on it. After some time I stop pushing the mass, from this moment till the mass stops what kind of friction works on the problem? (kinetic or static).

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Kinetic friction acts till the body is in motion. As soon as it stops, no friction acts on it unless pushed by an external agent with force F<= Max. Static Friction.

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  • $\begingroup$ "Kinetic friction acts till the body is in motion" - ? $\endgroup$ – lemon Jan 10 '17 at 10:48
  • $\begingroup$ Yes. Till the body is moving on the surface, kinetic friction acts on it. It is not a passive force $\endgroup$ – Pranshu Malik Jan 10 '17 at 10:49
  • $\begingroup$ If the body is not in motion then it's experiencing static rather than kinetic friction, by definition... $\endgroup$ – lemon Jan 10 '17 at 10:50
  • $\begingroup$ No. Static force is passive in nature, contrary to the "active" nature of kinetic friction. So what it means is that till you apply no force to the body at rest, no friction acts on it be it kinetic or static. Static friction only acts when we have an external agent pushing/pulling the body. When it is overcome, the body starts moving and kinetic friction starts acting till the body comes back to rest. $\endgroup$ – Pranshu Malik Jan 10 '17 at 10:53
  • $\begingroup$ No force, no movement = no friction. Force, no movement = static friction. Force or no force, movement = kinetic friction. "till the body is in motion" means it is presently not moving, so kinetic friction is not acting... $\endgroup$ – lemon Jan 10 '17 at 10:55

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