Question

A careless pilot leaves an empty coffee cup on top of his instrument panel as he accelerates along a horizontal runway. There is friction between the instrument panel and the coffee cup, but it is insufficient to hold the cup in place. The cup slides off the instrument panel and bangs into the pilot as he accelerates down the runway. 1)As it slides along the instrument panel toward the pilot, the work done by friction on the cup is negative. positive. zero. 2)The direction of the frictional force acting on the cup is towards the front of the plane. towards the back of the plane. undefined, since friction does no work in this case.

Answer 1

1) As it slides along the instrument panel toward the pilot, the work done by friction on the cup is positive.

Explanation: Frictional force acts in opposite direction to the direction of motion of the body. Friction can only be positive in that direction.

2) The direction of the frictional force acting on the cup is towards the front of the plane.

Explanation: as explained earlier, friction acts in opposite direction to the direction of motion.

The cup will move towards the rear of the plane due to inertial forces, friction will act towards the front of the plane to oppose this motion.