Speed

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

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_p is the change in gravitational potential energy in joules (J)
• – 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)