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Speed and velocity

What is the difference between " velocity " and " speed "?

Speed is a measure of how fast something is going.
Velocity is speed in a particular direction.

For example a car might have a speed of 20 m/s but a velocity of 20 m/s in a northerly direction.

The speed of the car would tell you how fast it was going but if you knew its velocity you could tell (for example) if it was going backwards or forwards. Its speed might be written as 10 m/s but its velocity would be written as + 10 m/s. A car going at the same speed in the opposite direction would have a velocity of 10 m/s.

You can see that velocity is a vital measurement for cars in traffic. The direction that they are moving in would make a real difference to the traffic flow!

We often use the word velocity without specifying the direction. This is just a rather bad habit but in problems where you actually use velocity the direction must be allowed for.

In a problem you can pick the positive direction it might be left to right or upwards. It does not matter as long as you stick to that direction throughout the problem.

© Keith Gibbs 2011