How do you calculate velocity?

What is the coefficient of kinetic friction μk between the block and the tabletop?

Engines for propeller-driven aircraft are limited in their maximum rotational speed by the fact that the tip speed of the propeller must not approach the speed of sound in air (mach i). taking 6 ft as a typical diameter for a propeller of a light airplane and 1100 fils as the speed of sound, find the upper limit on the rpm (revolutions per minute) of the propeller shaft.

How can a cyclist's acceleration change even if its velocity remains constant?