What is Momentum?

Example 25.7 (Average force, no details needed)

A $1300\,\mathrm{kg}$ car goes from rest to $100\,\mathrm{km}/\mathrm{h}$ ($27.8\,\mathrm{m}/\mathrm{s}$) in $8.0\,\mathrm{s}$: whatever the instant-by-instant story, the average net force is $\Delta p/\Delta t = 1300 \times 27.8/8.0 \approx 4.5 \times 10^{3}\,\mathrm{N}$ — momentum turns a force question into before-and-after bookkeeping.