Question

Two identical wooden blocks are tied one behind the other and pulled across a rough horizontal surface with the help of a horizontal force. If the force required to pull them at constant speed is F, and if we attach the block one over the other, the new force required to pull the blocks with constant speed is ___

Answer 1

1. The diagrammatic representation of blocks when tied one behind the other is given below:
2. 
3. The diagrammatic representation of blocks when attached one over the other is given below:
4. 
5. Force F is the force that is just required to overcome the frictional force. Hence no acceleration is produced.
6. In either case, the body moves with a uniform velocity when pulled with force F. That is because the mass of the system remains constant.
7. In the first configuration, friction acts in two places. In the second configuration, although the friction acts only in one place, the magnitude doubles due to both the masses being together. The total friction acting on the system remains the same.
8. Therefore for a different orientation of the same two masses, the frictional force remains the same.
9. Hence, it means that the amount of force required to just overcome the frictional force is the same as F.

Hence, the force required to move the body with uniform velocity is F.

The answer is Force F.