A rubber ball, travelling in a horizontal direction, strikes a vertical wall. It rebounds at right angles to the wall. The graph below illustrates the variation of the ball’s momentum \(p\) with time \(t\) when the ball is in contact with the wall.

Which of the following statements is true?

A.     The shaded area is equal to the force exerted by the wall on the ball.

B.     The shaded area is equal to the force exerted by the ball on the wall.

C.     The gradient is equal to the force exerted by the wall on the ball.

D.     The gradient is equal to the force exerted by the ball on the wall.

C

Most students understood that the gradient of the graph referred to a force, yet they failed to go one step further and identify the direction of the force. The gradient is negative, which means that it is in the opposite direction to the momentum of the ball. It can, therefore, only refer to the force by the wall on the ball.