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?
  • \(\frac{\left(500 \times 15 \right) - \left(500 \times 8 \right)}{5}\)
  • \(\frac{\left(500 \times 8 \right) - \left(500 \times 5\right)}{15}\)
  • \(\frac{\left(500 \times 8 \right) - \left(5 \times 15\right)}{500}\)
  • \(\left(500 \times 8 \right) - \left(5 \times 15\right) \times 500\)