# Decelerate

## Key Stage 3

### Meaning

Decelerate means to slow down.

Deceleration is the opposite of acceleration.
Deceleration is usually caused by air resistance, water resistance or friction.

## Key Stage 4

### Meaning

Deceleration is a negative acceleration in which the magnitude of the velocity decreases.

Deceleration is a vector because it has magnitude and direction.
The direction of deceleration is opposite to the direction of motion of an object.

### Rapid Deceleration

Rapid deceleration can be dangerous as the forces involved increase as the time taken to decelerate decreases.

The following equation shows how the force on an object during a deceleration is proportional to the change in momentum but inversely proportional to the time taken.

$$F = \frac{m \Delta v}{t}$$

Where

$$F$$ = The impact force.

$$m \Delta v$$ = The change in momentum.

$$t$$ = The time taken for the momentum to change.

When vehicles crash there is rapid deceleration so vehicles are designed to spread the change in momentum over the longest time possible to reduce the forces acting on the passengers.

The features that vehicles have to increase the time and therefor decrease the deceleration forces are:

• Crumple zones - Vehicles are designed so that the front will get crushed during the crash rather than being solid. This allows the change in momentum to take longer.
• Air Bags - Car's are often fitted with bags that will suddenly inflate in a crash and then deflate as the passenger collides with it. This allows the passenger's change in momentum to take longer.