Large decelerations

1. A deceleration happens whenever an object slows down, and a large deceleration means a large drop in velocity in a short time.
2. Newton's second law, $F=ma$, shows that a large deceleration can only be produced by a large resultant force.
3. For the same change in velocity, the shorter the time taken to stop, the larger the deceleration and so the larger the force.
4. In normal driving decelerations are small, but in emergency braking and especially in collisions they can be very large.
5. Large decelerations are dangerous in two ways, through the forces they produce on the people inside a vehicle and through their effect on the vehicle's brakes and tyres.

Forces on the body in a collision

1. When a car hits a solid object, the front of the car stops in a fraction of a second.
2. The people inside keep moving forwards at the car's original speed because of their inertia, until a force acts on them to slow them down.
3. If nothing slows them first, they hit the steering wheel, the dashboard or the windscreen and stop in a very short time.
4. The very short stopping time means a very large deceleration, so the force on the body is very large.
5. Large forces cause serious injuries, such as broken bones, damage to the chest and internal organs, and head injuries.
6. The brain is particularly at risk, because it keeps moving inside the skull when the head stops suddenly and can strike the inside of the skull.
7. Whiplash injuries happen when the body and the head change velocity at different moments, so the neck is bent sharply forwards or backwards.

Reducing the deceleration

1. Safety features cannot change the overall change in velocity in a crash, because the person still goes from moving to stopped.
2. What they can do is increase the time over which the person stops, which reduces the deceleration and so reduces the force on the body.
3. A longer stopping time also means a longer stopping distance, and transferring the same kinetic energy over a longer distance needs a smaller force.
4. Crumple zones at the front and back of a car crush in a controlled way during a collision, so the passenger compartment takes longer to stop.
5. Seat belts stretch slightly, so the wearer stops over a longer time, and they spread the force across the strong bones of the chest and hips.
6. Airbags inflate in a fraction of a second and then let gas out as the head and chest press into them, so the head stops gradually and does not hit a hard surface.
7. Cycle and motorcycle helmets contain a layer of foam that crushes on impact, so the head stops over a longer time.
8. Crash barriers at the side of roads bend and stretch when a vehicle hits them, so the vehicle stops over a longer time and distance.
9. Child car seats hold young children in the correct position, so the straps act on the strongest parts of their bodies.

Dangers during heavy braking

1. A large deceleration during braking needs a large braking force, and this force comes from friction between the tyres and the road.
2. The friction the road can provide has a maximum value, which is lower on wet, icy or loose surfaces.
3. If the driver brakes harder than this maximum allows, the wheels lock and the tyres skid over the road surface instead of rolling.
4. A skidding vehicle has less grip, so it takes longer to stop, and the driver can no longer steer it, which leads to a loss of control.
5. Most modern cars have anti-lock brakes, which release and reapply the brakes many times a second to stop the wheels locking.
6. Stopping quickly also transfers a large amount of kinetic energy to the thermal store of the brakes in a short time, so the brakes become very hot.
7. Overheated brakes produce less friction, so the vehicle takes longer to stop when it brakes again.
8. Loose objects and passengers without seat belts keep moving forwards during heavy braking and can hit the people in front of them.