Question

16.
AL
A disc revolves with a speed of 120 rev./min.
and has a radius of 15 cm. Two coins A and B
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 0.16, which
of the coins will revolve with the record ?
(1) A
(2) B
(3) Both
(4) None of them

Answer 1

Answer:

Here, mu=0.15,

v=33

3

1

rpm=

3

100

rpm=

3×60

100

rps=

9

5

rps

∴ω=2πv=2×

7

22

×

9

5

=

63

220

rads

−1

The coin will revolve with the disc, if the force of friction is enough to provide the necessary centripetal force.

i.e.

r

mv

2

≤μmg

As v=rω,∴mω

2

r≤μmg

For the given μ and ω, the condition is

r≤

ω

2

μg

As

ω

2

μg

=

(

63

220

)

2

0.15×10

=12cm

For coin A, r=4cm

The above condition is satisfied, therefore coin A will revolve with the disc.

For coin B, r=14cm

The above condition is not satisfied, therefore coin B will not revolve with the disc.

Note: we have nothing to do with the radius of the disc.

Answer 2

Answer:

b

Explanation:

because we have 14 cm,we- 15 and 14-15thats