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" does not accurately describe the situation of a ladder leaning against a smooth vertical wall. Here's why: When a ladder leans against a smooth vertical wall, there are no external horizontal forces acting on the ladder at its upper end. The wall provides a normal force perpendicular to its surface, but there is no frictional force present due to the smooth nature of the wall. However, the absence of a frictional force at the upper end does not mean that the force is zero. Rather, it means there is no force of friction acting horizontally at the upper end due to the smoothness of the wall. The ladder is free to slide horizontally along the wall without any resistance from friction at the contact point between the ladder and the wall. The ladder may still experience other forces, such as its weight and the normal force from the ground, but these forces act vertically and do not affect the absence of friction at the upper end. Therefore, while the statement "zero at its upper end" may imply the absence of a frictional force, it's more accurate to say that there is no horizontal frictional force due to the smoothness of the wall, rather than a complete absence of force.