14. Gradient of a distance-time graph

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Gradient of a distance-time graph

We can describe how an object moves with time by drawing a distance-time graph. Distance goes on the vertical axis and time is plotted on the horizontal axis.

For the straight line shown, as each second ticks by the distance away from the starting point increases by the same amount. This means that the object is moving at a constant speed and that's what a straight line means on a distance-time graph. The steeper the line is, the faster the object is travelling. Steepness is measured by the gradient of the line, so the larger the gradient, the faster the object. A useful equation to remember that links speed, distance and time is:

Average speed = distance travelled / time taken

What about a line on this graph which is 'flat'? In this case, as time ticks along the horizontal axis, there is no increase in the distance from the starting point. The object has stopped - or in physics 'speak', the object is stationary.

In GCSE Physics exam questions, it is really important that you talk about the motion of an object in 'physics language'. Yes, the object is moving, but how is it moving? An upwards straight line means that it is moving at a constant speed.

By the way - if the line is going downwards (a negative slope or gradient), then the object is still moving at a constant speed, but in the reverse direction (back to its starting point).

GCSE Physics keywords: Distance, Time, Speed, Gradient, Slope, Straight line, Constant Speed

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