Excellent! Force is change in momentum divided by the time taken. Momentum $$\vec{p} = m \vec{v}$$. So, we subtract the initial momentum from the final momentum, and divide this by the time taken.
The unit of Force is Newton (N). $$1 \, \mathrm{N} = 1 \, \mathrm{kg} \, \mathrm{m/s^2}$$.
If the $$500 \, \mathrm{kg}$$ lorry's velocity decreases from $$8 \, \mathrm{m/s}$$ to $$4 \, \mathrm{m/s}$$ in $$5 \, \mathrm{s}$$, what would be average Force acting on it?
• $$\frac{\left(500 \times 5 \right) - \left(500 \times 8 \right)}{4}$$
• $$\frac{\left(500 \times 8 \right) - \left(500 \times 5 \right)}{4}$$
• $$\frac{\left(500 \times 4 \right) - \left(500 \times 8 \right)}{5}$$
• $$\frac{\left(500 \times 8 \right) - \left(500 \times 4 \right)}{5}$$