The speed of an
object is a measure of how fast it is moving. It is given as the distance
the object would travel in a unit of time. So, for example, a car
which covers 18 miles at a constant speed on a motorway in 15 minutes,
has a speed of 72 miles per hour (72mph).
If an intercity train is traveling at a speed of 120mph then in 1
1/2 hours it will cover a distance of 120 x 1 1/2 = 180
miles. If an athlete averages a speed of 6.4 meters a second in a
race of 800 metres, she will have taken 800 ÷ 4 = 125 seconds
When the distance of an object has traveled is plotted against the
time it has taken, the slope or gradient of the graph at any point
on it gives the speed of the object at that time.
For example when a stone is thrown straight up with a speed of 20
ms-1 its height, h metres, above the point of projection after t seconds
is given by the formula h = 5t (4-t) and is shown on the graph. The
speed of the stone after 1.5 seconds is given by the gradient of the
tangent to the graph, shown in red, and is ms-1.