Why is it more difficult to revolve a stone by tying it to a longer string than by tying it to a shorter string?

 Answer: The Torque τ required to produce an angular acceleration α if given by τ = Iα, where (I=mr^2) is the moment of inertia. More is the distance of the stone from the axis of rotation, greater is its moment of inertia and hence more torque us required to produce same angular acceleration. Therefore, it is more difficult to revolve a stone by tying it to a longer string than by tying it to a shorter string.