# changing speed

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Acceleration is the rate at which an object changes its speed. It is calculated using the equation:

Acceleration = change in speed ÷ time taken.

Speed-time graphs illustrate how the speed of an object changes over time. The steeper the gradient of the line, the greater the acceleration.

Velocity is speed in a particular direction. An object accelerates if its velocity changes.

# Acceleration

In everyday language we use 'accelerate' to mean speeding up and 'decelerate' to mean slowing down. In scientific terms, positive acceleration means an increase in speed and negative acceleration means a decrease in speed.

Acceleration depends on two things:

• How much the speed changes
• How much time the change in speed takes

## Calculating acceleration

You can calculate acceleration using this equation:

Acceleration = change in speed ÷ time taken

For example, a bus increases its speed from 5 m/s to 25 m/s in 10s. What is its acceleration?

Change in speed = end speed – start speed = 25 – 5 = 20 m/s

Acceleration = 20 ÷ 10 = 2 m/s2

Notice that speed is measured in m/s but acceleration is measured in m/s2. You may find this triangle helpful when rearranging the equation.

A negative acceleration means that something has slowed down. For example, the bus decreases its speed from 25 m/s to 15 m/s in 5 s. What is its acceleration?

Change in speed = end speed – start speed = 15 – 25 = –10 m/s

Acceleration…