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Science

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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