Question

A cord is used to lower vertically a block of mass M by a distance d with constant downward acceleration (9/2). Work done by the cord on the block is 

(A) – Mgd/2 

(B) Mgd/4 

(C) – 3Mgd/4 

(D) Mgd

Answer 1

Correct option: (A) – Mgd/2

Explanation:

Tension in the cord = T = m (g – a)

here   a = (g/2)

T = m {g – (g/2)} = (mg / 2)

Workdone by cord = – Td

= {(– mgd) / 2}