Question

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. $Mg\frac{d}{4}$
• B. $3Mg\frac{d}{4}$
• C. $-3Mg\frac{d}{4}$
• D. $Mgd$

Answer 1

The correct option is C $-3Mg\frac{d}{4}$

Let $T$ tension in the sting

$\begin{array}{c}mg-T=\frac{Mg}{4}\\ T=Mg-\frac{Mg}{4}\\ =\frac{3Mg}{4}\end{array}$

Work done by tension

$\begin{array}{c}=-T\times d\\ =-\frac{3Mg}{4}\times d\\ =-\frac{3Mgd}{4}\end{array}$

Hence, Option $C$ is correct.