A cord is used to lower vertically a block of mass M by a distance d with constant downward acceleration …

Q: A cord is used to lower vertically a block of mass M by a distance d with constant downward acceleration g/4. Work done by the cord on the block is

(a) Mg d/4

(b) -3Mg d/4

(c) 3Mg d/4

(d) Mgd

Ans: (b)

Sol: During lowering a block, tension in rope is

T = m(g – a) and S = d

Work done W = – M (g – a)d

$\large W = – m (g – \frac{g}{4})d = – \frac{3 M g d}{4}$