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A 3 kg ball strikes a heavy rigid wall with a speed of 10 m/s at an angle of 60°. It gets reflected with the same speed and angle as shown here. If the ball is in contact with the wall for 0.2s, what is the average force exerted on the ball by the wall?

1. 150 N
2. zero
3. 150√3 N
4. 300 N

Correct answer: 150√3 N

Solution

The change in momentum occurs only in the horizontal direction (perpendicular to the wall). The initial horizontal momentum is 3 × 10 × cos(60°), and the final horizontal momentum is -3 × 10 × cos(60°). The change in momentum is thus 2 × 3 × 10 × cos(60°) = 30 kg·m/s. The average force is then Δp/Δt = 30/0.2 = 150√3 N.

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