Question

In which type of motion, we normally find the average speed of a moving body​

Answer 1

Answer:

A car moving with an average speed returns to the initial position

Suppose a car covers a distance of 50 m in 5 seconds and comes back to initial position in 5 seconds. The average speed of car is 10 m/s, but average velocity is zero because displacement is zero.

Answer 2

Speed and velocity are separate quantities. Speed is a scalar whereas velocity is a vector quantity. Although they are mostly confused with each other, a body can have different speed and different velocity cause the speed of a body depends only on its magnitude and the velocity depends both on the magnitude and direction of motion of the body.

Formula used:

s=dt

v=xt

Complete answer:

The speed of a body which is a scalar quantity is defined by the distance covered by the body in a particular stretch of time. Here, distance is also a scalar quantity.

s=dt

Where, s is the speed of a body

d is the distance

t is the time taken to cover the distance d

On the other hand, the velocity of a body is given by the displacement covered by the body in a particular stretch of time. Here, displacement is also a vector quantity.

v=xt

Where, v is the velocity of the body

x is the displacement covered by the body in

t is the time taken

Now, checking the options given, we have as follows:

(A)A car moving with an average speed returns to its original position.

In this situation, a car having an average speed covers a distance r(say) in time t. While returning back to the original position, it covers the distance r again in the same time t .Thus, the total speed of the car is:

s=r+rt⇒s=2rtm/s

The velocity is given by the total displacement covered in a given time. As the car returns back to its original position, the r is taken with a negative sign as displacement takes in account both magnitude and direction and since the direction of travel is opposite, we obtain the velocity as:

v=r+(−r)t⇒v=0m/s

Hence, in this case the average speed is non-zero while the average velocity is 0.

(B)A car at rest

A car at rest has both average speed and average velocity zero since it covers no distance and no displacement and is stationary.

(C)A car with non- zero displacement

A car with non-zero displacement will have both distance and displacement as non-zero. So both average speed and average velocity will be non zero in this case.

Hence, the correct option is (A).

A car moving with an average speed coming back to its original position will have zero average velocity and non-zero average speed since the displacement in this case is zero while the distance travelled is non-zero.

Additional Information:

Speed and velocity although might sound similar and we often use them in place of each other, in physical terms, both quantities have separate meanings. Although, even the units of speed and velocity are the same.

Note:

While solving the question, we must be careful to not confuse the terms distance and displacement. While solving for velocity, care must be taken during the use of sign (+ or -) which represents the direction of motion. The use of signs is completely ignored in case of average speed since speed is a scalar quantity.

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