Question

A particle of mass m is moving in yz-plane with a uniform velocity v with its trajectory running parallel to +ve y-axis and intersecting z-axis at z = a (Fig. 7.2). The change in its angular momentum about the origin as it bounces elastically from a wall at y = constant is:

(a) mva ê_x 

(b) 2mva ê_x 

(c) ymv ê_x 

(d) 2ymv ê_x

Answer 1

The initial velocity is v_i = vê_y and, after reflection from the wall, the final velocity is v_f = -vê_y . The trajectory is described as r = yê_y + aê_z . Hence the change in angular momentum is r x m(v_f - v_i) = 2mvaê_x.

Hence the answer is (b).