A penny is placed on a disk that is rotating at 2 revolutions per second. The coefficient of friction between the penny and the disk is 0.4. What is the maximum radius at which the penny will stay on the disk?

Question

a. 2.5cm
b. 100cm
c. 1cm
d. 10cm
e. 25cm

Answer 1

mu m g = m v^2/r = m omega^2 r

mu g = omega^2 r

omega = 2 *2pi radians/s = 4 pi rad/s
mu = .4
g = 9.81
so
r = .4 * 9.81 /16 pi^2
= .0248 meters = 2.5 cm