Velocity and displacement

In Physics we have to be a little more exact when talking about speed - the DIRECTION that something moves is just as important as how FAST it moves.

Imagine two photographs taken of a football match, one at the start and one at the end of the game. The referee may look as though he has only moved a few metres but in fact he will have run many kilometres during the game.

Think about a car starting off at a set of traffic lights. Moving FORWARDS at 2m/s may have a very different effect from moving BACKWARDS at 2m/s.

We use two new words to take into account this DIRECTION; they are DISPLACEMENT and VELOCITY

Displacement is measured in the same units as distance and velocity has the same units as speed.
Displacement is given the symbol s and velocity the symbol v and time the symbol t. So the formula for constant velocity is:

Constant speed and constant velocity

It is important to realise that there is a difference between CONSTANT SPEED and CONSTANT VELOCITY. If a car is moving in a straight line at a constant speed then its velocity is also constant but if it turns a corner, still keeping its speed the same, its velocity will have changed because the direction in which it was moving has changed.