Question

Question 4 Certain force acting on a 20 kg mass changes its velocity from 5 m s −1 to 2 m s −1 . Calculate the work done by the force.

Class 9 - Science - Work and Energy Page 158

Answer 1

Work done by the force is equal to change in the kinetic energy of the body

Now ,m=20kg

, u=5ms^−1 

,v= 2ms^−1,

W=?

Using the expresssions W= 1/2mv^2−1/2mu2 ,

We have W= 1/2m(v2−u2)=1/2×20((2)^2−(5)^2)

W= -210 J

Negative sign say that work has been done in slowing the body.

Answer 2

Heya,

Mass of the object = 20 kg

Initial velocity (u) = 5ms-1

Final velocity (v) = 2ms-1

Work done in this case is always equal to change in kinetic energy

i.e.

1/2 m(v)² - 1/2 m(u)²

=1/2 × 20 × (2)²-1/2 × 20 × (5)² 

=40 Joules - 250Joules

= -210 Joules

There is negative force acting on the object.

Hope it helps u.........