1. The problem statement, all variables and given/known data My problem is with the last line of the derivation. Usually, we take the rate of change of momentum (force) to be change of momentum divided by the impact time. But this slide of pp showed that the travelling time of the particle in the box is used but not the impact time. Why is it? Appreciate any comment on this. Thank you! 2. Relevant equations 3. The attempt at a solution I realised that if the impact time is used. The force will be very large since the impact time is likely to be very small.