A horizontal force of 80 N is required to push a bookcase across a floor at a constant velocity. How much friction acts on the bookcase when it is at rest on a horizontal surface without being pushed?

An important dimensionless parameter concerned with very high-speed flow is the mach number, defined as v/c, where v is the speed of the object such as an airplane or projectile, and c is the speed of sound in the fluid surrounding the object. for a projectile traveling at 1170 mph through air at 50 ˚f and standard atmospheric pressure, what is the value of the mach number?