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Once a crate is already sliding across the floor, how does the friction force acting on it behave if you keep pushing harder and harder?
AIt stays fixed at mu_k times the normal force
BIt keeps rising in proportion to how hard you push
CIt drops to zero because the crate is already moving
DIt jumps back up to the static friction maximum
Answer & Solution
Correct answer: A. It stays fixed at mu_k times the normal force
1. Once an object is moving, the magnitude of kinetic friction is given by a fixed equation, f_k = mu_k N.
2. That equation depends only on the coefficient of kinetic friction and the normal force, not on how hard you push.
3. So pushing harder does not raise the friction force once sliding has started, unlike the responsive static case.
4. Friction does not vanish just because the object is moving, and it does not jump back to the static maximum either.
_Source: OpenStax College Physics (CC BY 4.0), Ch 5 "Further Applications of Newton's Laws: Friction, Drag, and Elasticity", section 5.1 Friction_
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