Q1: On a ladder resting on a rough ground and leaning against a smooth vertical wall, the force of friction acts

ANS:D - zero at its upper end

"zero at its upper end." In the context of a ladder resting on a rough ground and leaning against a smooth vertical wall, the force of friction cannot be zero at its upper end, assuming there is contact between the ladder and the wall. Friction arises due to the interaction between surfaces in contact. In this case, the ladder's upper end is in contact with the smooth vertical wall, and friction would exist at this point to prevent the ladder from slipping down the wall. Therefore, the option "zero at its upper end" is not applicable in this scenario. Friction would indeed exist at the upper end of the ladder, acting upwards to oppose the ladder's tendency to slide down the wall.