# . Explain Work Done by Internal Force with diagram

i. Internal force exists for a system of particles. For a single particle, there is no internal force.

ii. Internal forces will do work when there is deformation within the system. In the case of rigid body net work done by internal forces is zero.

iii. Work done by internal force can be positive, negative or zero.

E.g. : In the case of compression, work done by internal forces will be negative; in the case of expansion work will be positive. When internal forces do positive work, energy of system decreases.

Suppose that a man sets himself in motion backward by pushing against the wall. The forces acting on the man are his weight W, the upward forces n1 and n2 exerted by the ground and the horizontal force N exerted by the wall. The works of W and of n1 and n2 are zero because they are perpendicular to the motion. The force N is the unbalanced horizontal force that imparts to the system a horizontal acceleration. The work of N, however, is zero because there is no motion of its point of application. We are therefore confronted with a curious situation in which a force is responsible for acceleration, but its work, being  zero, is not equal to the increase in kinetic energy of the system . The new feature in this situation is that the man is a composite system with several parts that can move in relation to each other and thus can do work on each other, even in the absence of any interaction with externally applied forces. Such work is called internal work. Although internal forces play no role in acceleration of the composite system, their points of application can move so that work is done; thus the man’s kinetic energy can change even though the external forces do no work.