How do simple machines make work easier? a. they add energy to the work. b. they change the strength or direction of a force. c. they reduce the speed of work. d. they increase the distance of applied forces.

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.