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

# Lessons from the drinking bird

## Introduction

The drinking bird is a common toy used for decoration in houses. Study of this toy gives an insight into many concepts of physics.

## Apparatus

The bird is made by joining two glass bulbs by a glass tube open at both ends. The upper bulb is covered by cotton cloth and the beak is made by a strip attached to the upper glass bulb again covered by cotton. A coloured liquid with a boiling point of around 42 degree celsius is filled inside the bird. The bird is pivoted in the middle, hence can oscillate about the support. When the bird is in its horizontal position, its beak goes inside a cup of water kept on the main frame. The bird then starts oscillating with large amplitude. As the oscillations die down the bird again dips its beak into the water cup and the whole process starts all over again.

## Procedure

1. Observe the motion of toy carefully.
2. In which direction does the liquid move when the bird is horizontal with its beak in water?
3. Observe the considerable amplitude of the oscillation as the bird starts oscillating. How do you account for this large amplitude?
4. See the liquid moving gradually from the lower bulb to the upper bulb. Is the liquid defying gravity?
5. Observe the damping of oscillations as the liquid moves up. Why did the oscillations die down?
6. When all the liquid moves to the upper bulb, the bird becomes vertical and then immediately drops to a horizontal position. Why?

## Discussion

The questions asked above involve various principles of physics. We will deal with them one by one

### When all the liquid moves to the upper bulb, the bird becomes vertical and then immediately drops to a horizontal position. Why?

Main reason: Rotational equilibrium

Since the bird is pivoted in the middle, it can only execute rotational motion about its support. If the weight of the bird on the either side of the support is same, the bird will be in a rotational equilibrium and the net torque will be zero. But if the weight of the bulbs (determined by the amount of liquid in them) is different, there will be a net torque which will cause the bird to tilt in the direction of the heavier weight and it will start oscillating .

Here when the liquid has moved to the upper bulb, and the oscillations die down, bird has becomes vertical. Although there is an uneven distribution of the weights about the support, the net torque has become zero. This is because the weight $$mg$$ of the upper bulb coincides with the vertical line passing through the centre of mass of the bird. So the bird is in a rotational equilibrium. But even a very slight disturbance causes the bird to fall from its vertical position and it becomes horizontal. So the equilibrium at the vertical position was an unstable equilibrium.

### Why does the liquid move from the lower bulb to the upper bulb?

Main reason: Creation of low pressure area in the upper bulb.

When the bird puts its beak in water, it is in a horizontal position. The liquid inside the bird moves to the lower bulb making it heavy. So the bird starts oscillating with large amplitude. But in this process the water on the beak starts evaporating by taking the energy from inside. This cools the gas inside the upper bulb. The vapour inside was already saturated at the room temperature. This is made possible by choosing a liquid with a boiling point near the room temperature. So lowering of this temperature causes some of the vapour to get condensed. This decreases the number of moles of the gas in the upper bulb. This decrease in the number of moles and the temperature lowers the pressure in the upper bulb causing the liquid to rise in the tube and bird gradually becomes vertical. But now the upper bulb is heavier than the lower bulb so the bird dips down to a horizontal position and its beak goes into water. In this position the gases in the lower and the upper bulb get mixed up equalising the pressure and the liquid moves to the lower bulb.

### When the bird starts oscillating from its horizontal position, it has considerable amplitude but the oscillations die down very soon. Why?

Main reason: Conservation of mechanical energy.

In the horizontal position the liquid moves to the lower bulb making it heavy so the bird falls down and attains a very high velocity and a high kinetic energy. Being pivoted in the middle the bird starts oscillating with high amplitude. But the gradual creation of low pressure in the upper bulb causes the liquid to rise to the upper bulb. As the liquid rises, the potential energy of the system increases and the kinetic energy of the bird decreases causing the oscillations to gradually die down.