Suppose a man is running and he gradually speeds himself up . For this he applies a force on the ground backward and the ground pushes him forward . This is probably due to to friction between the shoes of the man and the ground . The friction acting in this case is Kinetic friction which has a constant magnitude. Since the magnitude of frictional force is constant, how is the man able to accelerate himself ? Who is providing this force?
[Physics] How is a running man able to Accelerate himself
forcesfree-body-diagramfrictionnewtonian-mechanics
Best Answer
Don't think kinetic friction, just because some part of him is moving. Is it also kinetic friction if I stand still but swing my arms? Many particles in him or elsewhere might or might not move. They are irrelevant.
Only the particles in contact with the ground are relevant.
And they are not moving. His foot is not moving during the step. It is stationary and not sliding while touching. There is static friction here.
And static friction can vary easily.