**Speed**

The table below shows the distance a car travels over a certain period of time.

Distance (m) |
0 | 1000 | 2000 | 3000 | 4000 | 5000 | 6000 |

Time (s) |
0 | 50 | 75 | 100 | 125 | 150 | 175 |

This information can be represented in a graph.

This is known as a **distance-time graph** as distance is plotted against time. The slope of the line is the **speed**. As the slope gets steeper, so the speed increases.

The gradient of a distance-time graph can be used to figure out the speed at which an object was moving.

Using a distance-time graph you can find the speed from the following equation:

**speed = distance / time**

- – speed is measured in metres per second (m/s)
- – distance is measured in metres (m)
- – speed is measured in seconds (s)

**HIGHER TIER**

You should be able to calculate gradients on distance-time graphs.

To calculate the gradient of a line divide the change in the vertical axis by the change in the horizontal axis.

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**Gravitational potential energy**

On Earth **gravity** is a force which is constantly acting upon us. When we travel above the surface of the Earth we store potential energy called **gravitational potential energy**. This amount (while on Earth) depends on two factors:

**Mass**: the greater the mass of an object the higher the gravitational potential energy.**Height**above the ground: the higher an object is above the ground the more gravitational potential energy it has.

When an object is lifted up, for example if you pick a ball up from the ground, then work is done against gravitational force and the object gains gravitational potential energy.

The equation for calculating a change in gravitational potential energy is:

**E _{p} = m x g x h**

- –
**E**is the change in gravitational potential energy in joules (J)_{p} - –
**m**is the mass in kilograms (kg) - –
**g**is the strength of the gravitational field in newtons per kilogram (N/kg) - –
**h**is the change in height in metres (m)