Question

A spinning top has a ___ motion:
A. Rotatory
B. Circulatory
C. Linear
D. Straight line

Answer 1

Hint: Remember that a spinning top has a bottom tip about which it executes its motion. The top rotates about a straight line passing through its centre and maintains a constant tilt angle with this axis at all times. In other words, to get a top to spin, we provide it with the required spin momentum, and the bottom tip seems fixed in position to the surface in contact as its top moves in a circle, forming a spatial cone. This should help you classify its motion.

Complete answer:
Let us look at how a spinning top executes its motion.

A spinning top is designed in such a way that it has a squat body and a sharp point at the bottom. This means that when the top is at rest, it cannot stand on its own on the tip. The top is generally spun on its vertical axis, and the tip balances the top, which is attributed to the gyroscopic effect.

The gyroscopic effect is the tendency of a rotating body to maintain a steady direction of its axis of rotation. This results in the top describing a cone in space, where the bottom tip is the pointy end (constant end) of the cone, and the upper part is the end that rotates about the bottom tip’s axis in a circle such that the entire mechanism, if mapped out in space, looks like a cone. This is also referred to as precession. At any instant, the direction of this precession is determined by the torque exerted by its weight, and since torque is equal to the rate of change of momentum, the direction of the torque can be determined by the right hand rule where if you close your fist in the direction of rotation of the top, then your thumb signifies the direction of the torque that is acting on the top.

In conclusion, the torque acting on the centre of mass of the top provides the change in angular momentum of the top which facilitates its rotational procession about its vertical axis and helps it maintain balance while spinning.

After spinning for some time, the angular momentum will reduce, owing to the friction between the tip and the surface in contact with the tip which leads to an increase in precession, until the top can no longer balance itself, which is when it topples.

Thus, from this discussion we can easily conclude that a spinning top executes rotational motion since rotational motion is formally defined as the type of motion in which a body moves in a circle about a single line which is the axis of rotation. Since the motion of the spinning top also exhibits precession and forms a spatial cone, which means that the angle between the axis of rotation and the processing part of the top remains constant throughout, the rate of change of angular displacement remains constant, which is also a requirement for a body to be classified as executing rotational motion.

So, the correct answer is “Option A”.

Note:
It is interesting to see that the precession is faster and more pronounced as the top slows down. This means that the precession angular velocity is inversely proportional to the spin angular velocity.
Note that when a sufficient spin momentum is given to the top, it initially wobbles until the friction and the torque between the tip and the surface in contact with the tip force it to spin steadily about its axis. The same happens as the top slows down. It bobs up and down until it finally falls over. This is technically called a nutation, and is caused when the angle between the angular momentum vector and the axis of rotation begins to change as it succumbs to horizontal components of influencing forces (most often due to friction).