IIT JEE Physics (1978-2018: 41 Years) Topic-wise Complete Solutions

# Gyroscope using Toy Motor

## Objective

Make a gyroscope and learn multiple concepts

## Apparatus

toy motor, two CD, aluminium sheet, battery

## Procedure

The equipment consists of a toy motor powered by two AA batteries.

Making: Take two CD and an aluminium sheet of the size of CD. Place the aluminium sheet between two CD and join them with tape or adhesive. Fix this assembly to the axle of motor. Paste/draw Netwon's disc of seven colour on outer CD. This equipment can be used for three concepts as discussed below.

Newton Disc: When connected to battery, the motor start rotating and you get a low cost Newton's disc.

Magnetic Brake: Bring a magnet close to the rotating disc. The speed reduces. You can explain about eddy currents and magnetic brake.

Gyroscope: Suspend entire assembly with a thread fixed a point slightly away from the centre of mass of the assembly. When motor is switched on, the CD rotates providing angular momentum to the system. The torque due to weight about centre of mass causes angular momentum (direction) to change and the system precesses about vertical axis. Changing the polarity of battery change direction of rotation. Physical law is $$\vec{\tau}=\frac{\mathrm{d}\vec{L}}{\mathrm{d}t}$$. This set up is good for classroom demo as carrying bicycle wheel in classroom is little bit cumbersome.