# Bird in a box

There is a bird sitting on the base of an air filled box. The box is resting on the top of a top pan balance. At a certain moment the bird takes off and flies round at a constant level inside the box.

## Question:

Describe and explain the effect of this on the reading of the top pan balance starting with the moment the bird leaves the base of the box.

The box is now evacuated, the bird is given a space helmet fitted with breathing apparatus and suspended from the top of the box. The bird now starts to flap its wings.

As before describe and explain the effect of this on the reading of the top pan balance starting with the moment the bird leaves the base of the box.

(Thanks to Flora for the second part of the question)

In each case the weight of the bird must be supported.

In case 1 this is done by the bird flapping its wings. The bird accelerates upwards off the base of the box and so an additional force must be applied to give this initial acceleration – there is therefore an increase in the reading until the bird has reached level flight. Once this is achieved the reading on the balance oscillates about a steady value – this value being the weight of the bird. The amount of oscillation will depend on the rate at which the bird flaps its wings.

In case two the weight of the bird is supported by the string. The reading on the balance will show a steady value equal top the weight of the bird. As soon as the bird starts to flap its wings there will be an oscillatory reading as in case 1. To remain more or less at rest the bird must only flap its wings slowly otherwise it will start to rise.

Compare case 1 with that of a small helicopter in the box. The helicopter will remain hovering or in level flight due to the steady down draft of air produced by the rotors. In this case the reading on the balance will be constant.

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