Two lessons, one equation: $F \, \Delta t = \Delta p$ — the impulse-momentum theorem, read aloud F delta-t equals delta p. On the left, a force $F$ in newtons held for a time $\Delta t$ in seconds. On the right, $\Delta p$, the change in momentum in $\mathrm{kg \cdot m/s}$, which is $\Delta p = m \, \Delta v = m(v - u)$: the mass in kilograms times the velocity it gained or lost, where $u$ is the velocity before and $v$ the velocity after. Something brought to rest loses all of it, so $\Delta p = mu$ in size.

Work it in that order — momentum change first, force second — and the physics reads itself: $F = \dfrac{\Delta p}{\Delta t}$. Same $\Delta p$, a longer stop, a smaller force. One warning that costs more marks than any concept here: contact times arrive in milliseconds, and milli means a thousandth — $20\ \mathrm{ms} = 0.020\ \mathrm{s}$. Feed the raw 20 into the division and your force comes out a thousand times too gentle, which is exactly the direction that makes a dangerous design look safe. Estimate the order of magnitude first, every time; the estimate catches the slip before the marker does.