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.