A plate of mass `m`, length b, and breadth a is initially lying on a horizontal floor with length parallel to the floor and breath perpendicular to the floor. Find the work done to erect it on its breadth.

Explanation

Taking earth surface as reference point. Gravitational potential energy of the block at the position 1,

`U_1=mgh_1` (i)
Gravitational potential energy of the block at the position 2 is given as
`U_2=mgh_2` (ii)
Thus, work done by the external agent
`W=DeltaU=(U_2-U_1)=mgh_2-mgh_1`
where `h_2=b//2` and `h_1=a//2` as it is a regular-shaped object.
`implies W=mg(b/2-a/2)=(mg(b-a))/(2)`