Question

A cord is used to lower vertically a block of mass $M$, through a distance $d$ at a constant downward acceleration of $\frac{g}{8}$. Then the work done by the cord on the block is

• A. $\frac{Mgd}{8}$
• B. $\frac{3Mgd}{8}$
• C. $Mgd$
• D. $\frac{-7Mgd}{8}$

Answer 1

The correct option is D $\frac{-7Mgd}{8}$

$Mg-T=Ma=M\left(\frac{g}{8}\right)$

$\Rightarrow T=M(g-\frac{g}{8})=\frac{7}{8}Mg$
Work done by the cord : $W=T\times d\cos {180}^{\circ }$

$=\frac{7}{8}Mgd(-1)=-7Mgd/8$