A 0.150 kg cricket ball moves at 30 m/s towards a batsman and is struck straight back towards the bowler at 40 m/s. What is the change in momentum of the ball?

1. Momentum is a vector, so start by choosing a positive direction. Take the direction from the batsman towards the bowler as positive. 2. The initial velocity is 30 metres per second towards the batsman, which is minus 30 metres per second in this convention. 3. The final velocity is 40 metres per second towards the bowler, which is plus 40 metres per second. 4. The change in momentum is the mass multiplied by the change in velocity, so it is 0.150 kilograms multiplied by 40 minus minus 30 metres per second. 5. Forty minus minus thirty is 70 metres per second, so the change in momentum is 0.150 multiplied by 70, which is 10.5 kilogram metres per second. 6. The result is positive, so the change points from the batsman towards the bowler. 7. The 1.5 kilogram metres per second answer is the trap for subtracting the speeds as 40 minus 30 and ignoring the change of direction. 8. The 4.5 kilogram metres per second answer is only the initial momentum, 0.150 multiplied by 30, and the 6.0 kilogram metres per second answer is only the final momentum, 0.150 multiplied by 40. _Source: Siyavula Physical Sciences Grade 12 (Everything Science, CC BY 4.0), Chapter 2: Momentum and impulse_