Centripetal force on an object in circular motion is always directed in which direction?
a. toward the center of the circle
b. away from the center of the circle
c. in the direction of the object's velocity
d. in a direction opposite to the object's velocity

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.