Answer to Question #81118 in Mechanics | Relativity for gm

Question

suppose a wheel is turning at 100 rpm. a strobe light set at 50 flashes per minute is used to measure the wheel's angular velocity. why does the wheel appear to stand still even tho the two rates are not the same

Answer 1

Suppose the experiment is held in the dark room. An observer looks at the wheel and sees it only at moments when the strobe light flashes. Per one flash the wheel makes 100/50=2 complete rotations, which means that the observer sees the same position of the wheel with each flash. This phenomenon is known as a stroboscopic effect.
There is a simple rule: If the frequency of a rotating object is n times more than the frequency of a flash light (n – natural number), the object will seem motionless since the observer will see the object at the same position.