Describing Motion (AQA GCSE Physics Combined Science): Revision Notes

Speed and velocity

What are speed and velocity?

Speed and velocity both tell us how fast something is moving. However, they are not exactly the same thing.

Speed is how far an object travels in a certain amount of time. Speed only tells us how fast something is going. It doesn't tell us which direction it's moving in. Speed is called a scalar quantity because it has no direction.

Velocity is speed in a particular direction. Velocity tells us both how fast something is moving AND which way it's going. Velocity is called a vector quantity because it includes direction.

Key differences between speed and velocity

Here's an easy way to remember the difference:

• Speed = How fast (no direction needed)
• Velocity = How fast + Which direction

Units of speed

Speed can be measured using different units:

• Metres per second (m/s) - commonly used in physics
• Kilometres per hour (km/h) - used for cars and bikes
• Miles per hour (mph) - also used for vehicles

Typical speeds in everyday life

Here are some common speeds you might recognise:

• Walking: 1.5 m/s
• Running: 3 m/s
• Cycling: 6 m/s
• Cars on UK roads: 2 to 31 m/s
• Trains: up to 83 m/s in the UK
• Sound waves in air: 330 m/s
• Light waves: 3 × 10⁸ m/s (very fast!)

Calculating distance from speed

When an object moves at a steady speed, we can work out how far it travels using this equation:

If you need to find speed or time instead, you can rearrange this equation:

• Speed = $\frac{\text{Distance}}{\text{Time}}$ or $v = \frac{s}{t}$
• Time = $\frac{\text{Distance}}{\text{Speed}}$ or $t = \frac{s}{v}$

Average speed

Often, objects don't travel at exactly the same speed for their whole journey. When speed changes, we can calculate average speed.

$\text{Average speed} = \frac{\text{Total distance travelled}}{\text{Total time taken}}$

This gives us the steady speed the object would need to travel at to cover the same distance in the same time.

Understanding changing velocity

An object can have a constant speed but a changing velocity. This happens when the object keeps moving at the same speed but changes direction.

For example: A car going around a roundabout at 10 m/s has constant speed but changing velocity. This is because the direction keeps changing as it goes around the curve.

Worked examples