Question
A disc revolves with a speed of 
rev/min, and has a radius of 15 cm. Two coins are placed at 4 cm and 14 cm away from the centre of the record. If the coefficient of friction between the coins and the record is 0.15, which of the coins will revolve with the record?
Solution
Coin placed at 4 cm from the centre
Given,
Mass of each coin = mGiven,
Radius of the disc, r = 15 cm = 0.15 m
Frequency of revolution, ν =
rev/min=
=
rev/sCoefficient of friction, μ = 0.15
In the given situation, the coin having a force of friction greater than or equal to the centripetal force provided by the rotation of the disc will revolve with the disc. If this is not the case, then the coin will slip from the disc.
Case 1: Coin placed at 4 cm
Radius of revolution, r' = 4 cm = 0.04 m
Angular frequency, ω = 2πν
= 2 x
= 3.49 s-1
Frictional force, f = μmg
= 0.15 × m × 10
= 1.5m N
Centripetal force on the coin is,
Fcent. = mr'ω2
= m × 0.04 × (3.49)2
= 0.49 m N
Since f > Fcent, the coin will revolve along with the record.
Case 2: Coin placed at 14 cm
Radius, r" = 14 cm = 0.14 m
Angular frequency, ω = 3.49 s–1
Frictional force, f' = 1.5 m N
Centripetal force is given as,
Fcent. = mr"ω2
= m × 0.14 × (3.49)2
= 1.7m N
Since f < Fcent., the coin will slip from the surface of the record.
