The body 'A' is moving forward with certain acceleration over the ground. The ground applies a kinetic friction force on 'A' in backward direction to oppose the relative motion. The block 'A' also applies an equal and opposite force of friction on the ground, would that force of friction be kinetic or the static?

Intuitively, there is a relative motion of ground with respect to block, so, there must be kinetic friction. But since ground is a part of earth, an inertial frame in this case, why can't we term it as static friction?

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1 Answer 1


To determine whether friction is static or kinetic, examine whether the two surfaces are moving relative to each other -- if so, the friction is kinetic, and if not, it's static. It doesn't matter which inertial reference frame is chosen. Even though the ground is an inertial frame, the relative motion between the box and ground means that friction is kinetic here.

This makes sense in the context of the physical difference between kinetic and static friction. Static friction usually results from two surfaces "interlocking" together into a configuration that requires additional force to change. On the other hand, kinetic friction usually results from chemical interactions between the two surfaces. The interlocking can only occur between surfaces that aren't in relative motion.


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