5.9.5 Momentum & Safety

Safety Features (Physics Only)

• When braking hard, there is a large deceleration.
• A large force is felt by the passengers and the car.
• This can be dangerous, as the force felt can cause injury (e.g., neck whiplash).

Explanation

• This can be explained by the equation above:
  • Large deceleration = large change in momentum over a short time.
  • So, a large force is exerted on the object (person).

Safety Features:

Seatbelts

• Without these, when hard braking, you will keep moving and not decelerate, causing you to fly through the windshield.
• They strap you in but also stretch under large forces.
  • Stretching increases the distance moved slightly but extends the time taken for passengers to stop.
  • This decreases the rate of change of momentum and therefore reduces the force.

Crumple Zones

• Without these, the car would be a solid metal block, which would immediately stop during a crash instead of "softening" the blow.
  • These create slightly "softer" areas at the front of the car, which crumple upon a crash.
  • They absorb energy to deform and compact.
  • This increases the time taken for the car to stop.
  • This reduces acceleration and force on passengers.

Air Bags

• Without these, your head will whip forward during a crash, hitting the steering wheel or whipping back to hit the back of the head, which would cause serious neck energy.
  • These inflate instantaneously upon a crash.
  • Your head hits this and slows down.
  • Increases the time taken for the head to stop moving.
  • So reduces the force on the neck.