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Changing speed

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# Velocity

The velocity of an object is its speed in a particular direction.

This means that two objects could be travelling at the same speed but have different velocities. For example, two cars are travelling at 30 m/s along the same road but in opposite directions:

• One of the cars has a velocity of +30 m/s
• The other car has a velocity of –30 m/s

The opposite signs show that they are travelling in opposite directions.

## Relative velocities

If two objects are moving in parallel their relative velocity can be calculated.

For example, two cars are moving in the same direction along a road. Car A is travelling at +30 m/s and Car B is travelling at +20 m/s. Their relative velocity is 30 – 20 = +10 m/s.

If the two cars are moving in opposite directions, the velocity of Car A is +30 m/s and the velocity of Car B is –20 m/s. Their relative velocity is 30 –(–20) = 30 + 20 = +50 m/s.

## Change in velocity - Higher tier

Acceleration is a change in velocity. This means that an object accelerates if:

• Its speed changes
• Its direction changes
• Both its speed and direction change

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