Students should be able to explain the dangers caused by large…

The idea

When a vehicle stops quickly, its occupants and the vehicle itself must lose momentum in a short time. A large deceleration means a large rate of change of velocity, so a large resultant force acts. The force on a passenger is exerted through the seat belt, and the pressure on the body can cause injury. Brakes convert kinetic energy into thermal energy; very large decelerations can overheat brakes and reduce their effectiveness, increasing stopping distance. The relationship F = m × a shows that for a fixed mass, a larger deceleration requires a larger force.

For example: A car of mass 1200 kg decelerating at 8 m/s² experiences a resultant force of 9600 N, which is dangerous without a seat belt.

What earns the marks

• State that large deceleration means velocity changes greatly in a short time, so the rate of change of velocity is large.
• Use F = m × a to explain that a larger deceleration produces a larger resultant force on the vehicle and its occupants for a given mass.
• Explain that a seat belt or crumple zone increases the time taken to decelerate, reducing the deceleration and therefore the force on the passenger.
• Describe that brakes transfer kinetic energy to thermal energy, and very large decelerations can cause brakes to overheat and fade, reducing braking force.
• Link injury to the pressure or force exerted on the body when a passenger is stopped by a hard surface rather than by a belt or airbag.

Common mistakes and how to put them right

• Thinking that a large deceleration means a large speed.

  Put it right by stating that deceleration is the rate of change of velocity, so a vehicle can have a small speed but still decelerate rapidly.

• Believing that the force on a passenger is the same whatever the stopping time.

  Put it right by explaining that for a fixed change in momentum, a shorter stopping time gives a larger force.

• Assuming brakes always work perfectly.

  Put it right by explaining that overheating can reduce braking force and increase stopping distance.

Exam tips

• Always link the size of the deceleration to the size of the resultant force using F = m × a, and name the force on the passenger.
• When explaining safety features, state clearly that they increase the time of deceleration, which reduces the deceleration and the force.
• Use the correct units for deceleration (m/s²) and force (N), and show any substitution clearly if a calculation is required.