Question

Two identical 5 kg blocks are moving with same speed of 2 ms^-1 towards each other along a frictionless horizontal surface. The two blocks collide, stick together and come to rest. Consider the two blocks as a system. Calculate work done by (i) the external forces, and (ii) the internal forces.

Answer 1

As no external force is applied on the system,

vector F_ext = 0

W_ext = vector(F_ext. x S) = 0

According to the work-energy principle, total work done = change in K.E. = Final K.E. - Initial K.E.

Negative sign implies that internal force of action and reaction act on the two blocks in a direction opposite to their motion.