Explain why the force of friction is not always equal to μ N; include static vs kinetic regimes.

Explain why the force of friction is not always equal to μ N; include static vs kinetic regimes.

• A. Static friction can take any value up to μ_s N to oppose motion; kinetic friction is a constant μ_k N when sliding
• B. Static friction is always less than μ_s N
• C. Friction is always equal to μ N
• D. Friction is independent of the contact area

Answer: (A)

The key idea is that friction is not a single fixed value μN in all situations. There are two regimes to consider: when the contact is at rest (static) and when it is sliding (kinetic). In the static case, friction acts to oppose any tendency of the surfaces to slide past each other. It can take on a range of values from zero up to a maximum of μsN. If you gently push, the friction force increases just enough to cancel your push, so the block stays put, and the actual friction is often less than μsN. Only when your push reaches that maximum μsN does the block begin to move. Once motion starts, the friction force enters the kinetic regime and becomes approximately a constant μkN, which is typically smaller than μsN. This is why when sliding, friction doesn’t keep increasing with your push; it settles into that steady value as long as sliding continues. So friction isn’t always equal to μN because the appropriate μ depends on whether the object is stationary or sliding, and the static maximum μsN is generally larger than the kinetic value μkN.

The key idea is that friction is not a single fixed value μN in all situations. There are two regimes to consider: when the contact is at rest (static) and when it is sliding (kinetic).

In the static case, friction acts to oppose any tendency of the surfaces to slide past each other. It can take on a range of values from zero up to a maximum of μsN. If you gently push, the friction force increases just enough to cancel your push, so the block stays put, and the actual friction is often less than μsN. Only when your push reaches that maximum μsN does the block begin to move.

Once motion starts, the friction force enters the kinetic regime and becomes approximately a constant μkN, which is typically smaller than μsN. This is why when sliding, friction doesn’t keep increasing with your push; it settles into that steady value as long as sliding continues.

So friction isn’t always equal to μN because the appropriate μ depends on whether the object is stationary or sliding, and the static maximum μsN is generally larger than the kinetic value μkN.