Question

Which one of the following is most probably not a case of uniform circular motion?
A) Motion of a racing car on a circular track.
B) Motion of the moon around the earth.
C) Motion of toy train on a circular track.
D) Motion of seconds hand on the circular dial of the watch.

Answer 1

Hint : Keep in mind that uniform circular motion is the motion where a body is moving with constant speed along a circular path. It is not a body moving with constant velocity because here the speed is constant, but velocity changes at every point because the magnitude of direction changes at each point of time.

Complete step-by-step solution:
Option (b)
Gravitational attraction provides the centripetal force needed to keep planets in orbit around the sun and all types of satellites in orbit around the earth. The Earth’s own gravitational force keeps the Moon orbiting around it. It keeps changing the direction of the Moon’s velocity hence the velocity of the Moon keeps changing, but it’s speed remains constant.
So we can say that Moon definitely follow uniform circular motion
Option (c)
In the case of a toy train, the device has a pre-set value of speed at which it operates, which in most cases, is not change-able. So if a toy train is moving in a circular track, even if if’s magnitude of direction keeps on changing, the speed remains the same and its acceleration directed inwards.
Hence we can conclude that toy trains on a circular track can also be considered uniform circular motion.
Option (d)
The seconds hand on the clock covers a fixed distance at every second, i.e. at regular intervals of time. So it has a constant speed. But since the angle changes at every point of time, the magnitude of direction does not remain constant therefore the velocity magnitude does not remain constant.
Hence we can say that the seconds hand on the clock follows uniform circular motion.
In option (a) we have a racing car on a circular track. This is the case most likely to not fall in the category of uniform circular motion because it is firstly a racing track, hence speed remaining constant at all points of time is not guaranteed, and secondly during sharp curves, it is necessary to decrease speed a little bit to avoid mishaps like toppling.
So we can conclude by saying option (a) is the correct answer.
Note: Problem of toppling is one prevalent at steep turns, especially if the vehicle is moving at high speed. It is due to the shift of center of gravity from its mean position. Usually to prevent topping, roads are banked; meaning the edges are raised at the inner edge to provide the vehicle ample force to take a safe turn. Usually this is absent in race tracks.