A horizontal cylinder has two sections of unequal cross - sections, in which two pistons can move freely. The pistons are joined by a string. Some gas is trapped between the pistons. If this gas is heated, the pistons will

Considering the same pressure along the two inside surfaces of the piston.

We have two different areas over there the left one being smaller and the right one larger.$\text{F on the inner left surface }=\mathrm{P}\times \mathrm{a}$

$\mathrm{F}\text{ on the inner right surface }=\mathrm{P}\times \mathrm{A}$

Force is proportional area. Hence force on right side is more.