Crumple zones
Newton's Laws & Momentum - OCR A-Level Physics
- Crumple zonesRegions of a vehicle designed to deform in a collision, increasing the time of impact and reducing the force.: the front and rear of a car deform on impact, increasing the collision time and reducing the peak force on passengers
- AirbagsSafety devices that inflate rapidly in a collision to increase the time of deceleration, reducing the force on the occupant.: inflate to cushion the occupant, spreading the force over a larger area and increasing the deceleration time
- SeatbeltsRestraints that spread the decelerating force across the chest and pelvis and extend the time of deceleration.: stretch slightly to increase the time over which the passenger decelerates, reducing the force. They also prevent the passenger from striking the dashboard
- All three work on the same principle: F = $\Delta$p / $\Delta$t. For the same change in momentum, a longer time means a smaller average force
- Helmets work similarly: the inner foam compresses on impact, increasing contact time
Common Mistake
MEDIUM
Students often: A typical mistake is to write that crumple zones 'absorb energyThe capacity to do work. Measured in joules (J).' without linking to the impulseThe product of force and the time for which it acts. Equal to the change in momentum. equation.
Instead: The key physics: crumple zones increase the TIME over which the passenger decelerates. Since $impulseThe product of force and the time for which it acts. Equal to the change in momentum. = F x \Delta t = \Delta p (which$ is fixed), a longer Delta t means a smaller force F on the passenger.
Instead: The key physics: crumple zones increase the TIME over which the passenger decelerates. Since $impulseThe product of force and the time for which it acts. Equal to the change in momentum. = F x \Delta t = \Delta p (which$ is fixed), a longer Delta t means a smaller force F on the passenger.