I might be asking a very elementary question here. How do we identify where static friction is acting or kinetic friction is acting?
For example, Consider this case:
Now, here we are talking about "sliding" a body along the ground with the least possible force.. If we are talking about sliding , then that would mean it will involve relative motion between the ground and the body.. causing kinetic friction. As $\mu_s$=$\mu_k$ , it does not cause much of a problem. However in the solution , it was given as follows :
When the block is about to start sliding, the frictional force acting on block reaches to its limiting value
f =$μ_s$N
Why do we not directly take the case of sliding friction here? I mean , if $\mu_s$ would not have been given equal as $\mu_k$, then why would i go with limiting case instead of directly considering the case where the object is sliding?
Best Answer
In general, static friction is greater than kinetic friction. That can be easily shown by placing an object on a flat surface (ceramic surface), and gradually increasing the angle of incline of the surface. A moment comes when the object starts moving downwards.
If kinetic friction were equal to the static one, we would see velocities as small as we wished, by inclining the surface very slowly. But experience tells us that when the static friction is overcome, the acceleration $a = mg\ \sin(\theta) - f_a$ jumps from zero to a finite value well above zero.
So, $f_a$ just after the object starts to move is smaller than it was before.