Question

A cord is used to lower vertically a block of mass $m$ by a distance d with constant downward acceleration $\frac{g}{4}$ Work done by the cord on the block is

• A. $\frac{3}{4}mgd$
• B. $-\frac{3}{4}mgd$
• C. $mgd$
• D. $\frac{mgd}{4}$

Answer 1

Answer: (B)

$T=m(g-a)$ (downward)
$T=m\left(g-\frac{g}{4}\right)=\frac{3mg}{4}$
$\therefore$ work done by the cord $=\vec{F}\cdot \vec{s}\cos \theta$
$=\frac{3mg}{4}\times d\cos 180^{\circ }$
$W=\frac{-3mgd}{4}J$