Question

A force acting on a 20kg mass changes its velocity from 5 m/s to 2 m/s . Calculate the work done by the force

Answer 1

By conservation of energy, Work done by force = Change in KE of system

Change in KE= 1/2×m×(v^2-u^2) = 1/2 × 20 × (2^2 - 5^2) = -210 J

Work done by force is 210J.
The minus sign indicates that the force was acting in the opposite direction of motion.

Answer 2

Solutions:

We know that whenever work is done, an equal amount of energy is used up. So, the work done in this case will be equal to the change in kinetic energy of bicycle when its speed changes from 2 m/s to 5 m/s.

(a) In the first case;

Mass of bicycle, m = 20 kg

And, speed of bicycle, v = 2 m/s

So, kinetic energy, Ek = 1/2mv^2

=> 1/2 × 20 × (2)^2

=> 10 × 4

=> 40 J

(b) In the second case;

Mass of bicycle, m = 20 kg

And, speed of bicycle, v = 5 m/s

So, kinetic energy, Ek = 1/2mv^2

=> 1/2 × 20 × (5)^2

=> 10 × 25

=> 250 J

Now, Work done = Change in kinetic energy.

=> 250 - 40

=> 210 J

Thus, the work done is 210 J.