Very good! The equation is $$\vec{p} = m \vec{v}$$. Multiplying the mass and the velocity gives us the momentum.
Newton's Second Law of Motion tells us that the force exerted on a body equals the rate of change of its momentum. That is, force $$\vec{F} = \frac{\text{Final Momentum} - \text{Initial Momentum}}{\text{Time Taken}}$$. So, you take the change in momentum, divide it by the time and you get the force exerted.
Our $$500 \, \mathrm{kg}$$ lorry was travelling at $$8 \, \mathrm{m/s}$$. The lorry driver presses the accelerator and the velocity increases to $$15 \, \mathrm{m/s}$$ in $$5 \, \mathrm{seconds}$$. What is the average force that was applied on the lorry?
• $$\frac{\left(500 \times 15 \right) - \left(500 \times 8 \right)}{5}$$
• $$\frac{\left(500 \times 8 \right) - \left(500 \times 5\right)}{15}$$
• $$\frac{\left(500 \times 8 \right) - \left(5 \times 15\right)}{500}$$
• $$\left(500 \times 8 \right) - \left(5 \times 15\right) \times 500$$