(a) Certain force acting on a 20kg mass changes its velocity from 5m /s to 2 m/s . calculate the work done by the force .

(b) State two possible condition under which the work done would be zero.

(c) if the kinetic energy of the body is increased by 300% then determine the percentage increase in the momentum.


Mass of an object m= 20 kg,Initial velocity u = 5m/sand final velocity v = 2m/s

.·. Initial kinetic energy of object E1 = ½ mu²= ½ x 20 x (5)² = 250 J 

And final kinetic energy of object E2 = ½ mv²= ½ x 20 x (2) ² = 40 J 

.·. Loss in kinetic energy = E1 – E2 = 250 – 40 = 210 J 

Work done by the force W = Change in kinetic energy = -210 J

The negative sign of work shows that there is decrease in kinetic energy of the mass. 


(i) There is no net force applied to the object.

(ii) Force is being applied but it does not cause any displacement.

(iii)Force is being applied and the object suffers displacement too but angle between the force and displacement is 90°.


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