# Distance and Speed

The distance travelled by a body is the actual length of the path covered by a moving body, not taking into account the direction in which the body travels. Different objects may take different amounts of time to cover the same distance. Some may move fast while others may move slowly. To measure the rate of motion of an object, we should find out the distance travelled by the object in unit time. This quantity is known as speed**of the object.**

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The speed of the body is the distance travelled in unit time.

The SI unit of speed is m/s or ms^{-1}. Distance and speed are scalar quantities.

The speed of an object need not necessarily be constant, as in most cases, the objects exhibit non-uniform motion. For such objects we describe the rate of change of motion as the average speed.

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Average speed = Total Distance travelled / Total Time taken

If the object moves a distance â€™sâ€™ in time â€˜tâ€™ seconds, its speed â€˜vâ€™ = s / t.

When the same body moves from one position to another, the shortest(straight line) distance between the initial position and the final position of the body, along with the direction gives us its displacement.

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Consider an object moving from O to A through C and B. â€˜Oâ€™ is the reference point. Here the magnitude of the distance travelled and the magnitude of displacement are the same, i.e. equal to 60 km. Having reached A, it starts moving back to C through B. Now, the total distance it has moved is 60 + 35 = 95 km.

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What is its displacement?

When the object moved back from A to C, the total distance it had travelled was 95 km, but the displacement is only 25 km. You see here that the magnitudes of distance and displacement are not equal.

Had the object moved back to O, the displacement would have been zero, as the initial and the final positions coincided. But the distance travelled would have been 60 + 60 = 120 km.