Measuring the rate of motion

Tanusri Gururaj, Academic content writer of Physics at Edumarz

Objects are not always in uniform motion where they cover equal distances in equal intervals of time. Hence, we can use the concept of average speed and average velocity.

Average speed

  • To calculate the average speed, the total distance covered is divided by the total time taken.

  •  It is a scalar quantity and only has magnitude.

  • Average speed = Total distance/Total time 

  • Unit of average speed = m/s

Take an example of a bus that covers 8m in 4 seconds, 4m in 2 seconds, and 10m  in 7 seconds.

Since the direction is not specified, we will use the formula for average speed.

For this question,

Average speed = ((8+4+10)/(4+2+7)) m/s

Average speed = 22/13 m/s = 1.692 m/s

Average velocity

  • If the average velocity of an object changes at a uniform rate then it is calculated by taking the mean of initial velocity and final velocity. 

  • It is a vector quantity and has direction as well as magnitude.

  • Average velocity = (Initial velocity + Final velocity)/2

  • Unit of average velocity = m/s

Here is an example to understand the difference between the two.

Suppose a cycle starts from point A, goes to point B, and returns to initial position A.

In this case, the average velocity will be 0 as the total displacement = final position – initial position and the initial and final positions are the same.

But, the average speed will not be 0 as the total distance will be 2AB.

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