Question

When is a body said to be in motion?
A. A body is said to be in motion when its position continuously changes with respect to time
B. A body is said to be in motion when its position doesn’t change with respect to time
C. A body is said to be in motion when its average speed is zero.
D. All the above.

Answer 1

Hint: Velocity is a vector quantity while speed is scalar quantity. Velocity depends on direction. Speed is independent of direction. Speed is defined as distance upon time in no particular direction. Velocity is the rate of change of distance in a particular direction.

Complete answer:
A body is said to be in motion when its position continuously changes with respect to time.
When the average speed is zero then the body is not in motion. Also when the position doesn’t change with time, the body is said to be at rest.

So, the correct answer is “Option A”.

Additional Information:
When a body is in motion, it moves in a straight line with constant velocity thus covering equal distance in equal interval of time irrespective of the duration of time is known as uniform motion.
When a body is in motion, it moves in different directions with changing velocities thus covering different distances in different intervals of time, irrespective of the time interval duration is known as non-uniform motion.
Acceleration of a body is non-zero only if it moves with varied velocity over a given interval of time.

Note:
Students usually get confused between uniform motion and uniform acceleration. Uniform acceleration leads to non-uniform velocity and is not the same. In uniform motion, a body travels with a constant velocity. In uniform acceleration, a body travels with constant increase or decrease in velocity. Students might think the above statement is false if they think of speed and velocity to be the same but they differ on the basis of direction.