5.37 A disc revolves with a speed of 3313 rev/min, and has a radius of 15 cm. Two coins are placed at 4 cm and 14 cm away from the center 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?

The coin revolves with the record in the case when the force of friction is enough to provide the necessary centripetal force. If this force is not sufficient to provide centripetal force, the coin slips on the record. Now, the frictional force μR where R is the normal reaction,

and R = mg
Hence the force of friction = μmg and centripetal force required is mυ2/r  or mrω2 are the same for both the coins and we have different values of r for the two coins. So to prevent slipping i.e. causing coins to rotate,μmgμrω2
Or, mgrω2

 


For first coin:
r=4 cm =4100m
n=3312 rev/min=1003×60 rev/s
ω=2πn=2πn=2π×100180=3.49 s-1
rω2=4100×3.492=0.49 ms-2
And, μg=0.15×10=1.5 ms-2
as μg>rω2
The coin will revolve with record.
For second coin:
r=14 cm =4100 m
ω=3.49 s-1
 rω2=14100×3.492
=1.705 ms-2
And, μg=1.5 ms-2
as  μg<rω2
The coin will not revolve with record