A rectangular block of mass `M` and height a is resting on a smooth level surface. A force `F` is applied to one corner as shown in Fig. At what point should a parallel force `3F` be applied in order that the block shall undergo pure translational motion? Assume normal contact force a between the block and surface passes through the centre of gravity of the block.

Options

A. `a/3` vertically above centre of gravity
B. `a/6` vertically above centre of gravity
C. no such point exists
D. it is not possible

Answer

B

Explanation

The free body diagram of the block can be drawn as shown.
As the body has to move in pure translational motion, the torque about the centre of gravity must be zero.

`3Fxxx=Fxxa/2impliesx=a/6`