A cord is used to lower vertically a block of mass `M`, a distance `d` at a constant downward acceleration of `(g)/(4)`, then the work done by the cord on the block is

Options

A. `Mg(d)/(4)`
B. `3Mg(d)/(4)`
C. `-3Mg(d)/(4)`
D. Mgd

Answer

C

Explanation

When the block moves vertically downward with acceleration `(g)/(4)` then tension in the cord
`T=M(g-(g)/(4))=(3)/(4)Mg`
Work done by the cord `= vec(F).vec(s)=Fs cos theta`
`= Td cos (180^(@))=-((3Mg)/(4))xx d = -"3 Mg "(d)/(4)`