Explain the dangers caused by large decelerations WS 1.5

The idea

Large decelerations happen when a vehicle loses speed very quickly, for example in an emergency stop or a collision. The deceleration is caused by a large resultant force acting opposite to the motion. This force acts on the vehicle and on the people inside it. Because the people have inertia, they continue moving forward until a force acts on them, so they may hit the interior of the vehicle or a seat belt. Large decelerations can also cause the vehicle to skid if friction between tyres and road is insufficient, and they increase the risk of injury because the forces on the body are large. Safety features such as seat belts, air bags and crumple zones increase the time taken to stop, which reduces the deceleration and the forces on the occupants.

What earns the marks

• State that large decelerations involve a large change in velocity over a short time.
• Explain that a large deceleration requires a large resultant force acting on the vehicle and its occupants.
• Describe how inertia causes passengers to continue moving forward when the vehicle decelerates rapidly.
• Identify dangers such as head or chest injury from impact with the steering wheel, dashboard or seat belt.
• Explain how safety features increase the stopping time and therefore reduce the deceleration and the forces on the body.
• Recognise that skidding can occur when large braking forces exceed the available friction between tyres and road.

Common mistakes and how to put them right

• Thinking that passengers are thrown forward by a force pushing them.

  Put it right: They continue moving due to inertia while the vehicle slows down.

• Believing that a larger deceleration reduces the forces on the body.

  Put it right: A larger deceleration means a larger resultant force, which increases injury risk.

• Confusing deceleration with negative acceleration in a way that ignores direction.

  Put it right: Deceleration is acceleration opposite to the direction of motion, so it reduces speed.

Exam tips

• Link every danger to a cause, such as inertia, large resultant force or insufficient friction, rather than listing dangers alone.
• When explaining safety features, state clearly that they increase the time taken to stop, which reduces deceleration and force.
• Use the equation F = m × a to support your reasoning about how force, mass and deceleration are related.