Question

A ball of mass 4 kg is moving with a speed of 10 m/s. Its speed reduces to 5 m/s due to frictional force. The work done by frictional force is
(1) 150J
(2)-150J
(3) 75J
(4) -75J​

Answer 1

Answer:

2.-150 J

Explanation:

Given:

m=4 kg

u=10 m/s

v=5 m/s

Solution:

Various forces acting on the body are:

1. Normal reaction in vertically upward direction
2. weight of the body in vertically downward direction
3. Force of friction in the opposite direction of motion

The change in velocity is only due to  frictional force cause weight and normal reaction are balanced and contribute nothing in changing the velocity as they are acting in a different plane.

mg=N   (in vertical direction)

From work energy theorem we know that

Work done by all the forces=change in kinetic energy

W=ΔKE

=KEf - KEi

KEf=1/mv²

=1/2*4*5²

=50 J

KEi=1/2mu²

=1/2*4*10²

=200 J

therefore

W=50-200

=-150 J

Hence frictional force does -150 J of work.

PS: the negative sign indicates that force and displacement are in opposite direction.

Hope this helps

Answer 2

u=10m/s

v==5m/s

v^2-u^2=2as

25-100=2×9.8×s

s=-750/196

w=f .s.

w=4×9.8×-750/196

w=-150