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Explain Clearly, with Examples, the Distinction Between Magnitude of Average Velocity Over an Interval of Time, and the Average Speed Over the Same Interval - Physics

Explain clearly, with examples, the distinction between magnitude of average velocity over an interval of time, and the average speed over the same interval. [Average speed of a particle over an interval of time is defined as the total path length divided by the time interval]. Show in both (a) and (b) that the second quantity is either greater than or equal to the first.

When is the equality sign true? [For simplicity, consider one-dimensional motion only].

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Solution 1

Magnitude of average velocity = `"Magnitude of displacement"/"Time interval"`

For the given particle.

Average velocity =  AC/t

Average speed = `"Total path length"/ "Time interval"`


Since (AB + BC) > AC, average speed is greater than the magnitude of average velocity. The two quantities will be equal if the particle continues to move along a straight line.

Solution 2

Suppose, in the above example, the particle takes time t to cover the whole journey. Then, the magnitude of the average velocity of the particle over time-interval t is = Magnitude of displacement /Time-interval =0/t =0

While the average speed of the particle over the same time-interval is =Total path length /Time-interval= 2 AB /t

Again, the second quantity (average speed) is greater than the first (magnitude of average velocity).

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NCERT Class 11 Physics Textbook
Chapter 3 Motion in a Straight Line
Q 13.2 | Page 57
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