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 Question: What is Speed ? Answer: 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.