Resonance (Edexcel A Level Physics): Exam Questions

Exam code: 9PH0

21 mins6 questions
1
6 marks

The photograph shows a ‘singing bowl’.

Photograph of a metal singing bowl: a wide, shallow bowl with a flared rim decorated with a patterned band, and two upright loop-shaped handles fixed to opposite sides of the rim.

When the handles are rubbed with both hands the bowl ‘sings’, producing a loud note with a frequency of 720 Hz.

A vibration generator is attached to the bowl and connected to a signal generator. The signal generator is adjusted to produce frequencies from 600 Hz to 800 Hz.

At all frequencies in this range the bowl produces a sound at the applied frequency. The sound is quiet for all frequencies except 720 Hz, when it is much louder.

Explain these observations.

2a
2 marks

A washing machine rests on spring mounts. During the spin cycle, the wet clothes in the rotating drum are not evenly distributed, so the drum exerts a periodic force on the machine. The frequency of this force is equal to the rotation frequency of the drum.

The machine and its contents can be modelled as a mass–spring system oscillating vertically. The mounts obey Hooke's law.

Explain what is meant by resonance.

2b
5 marks

The mass of the empty washing machine is 72 kg. When wet clothes of mass 8.0 kg are placed in the drum, the machine sinks a further 2.0 mm on its mounts.

Calculate the rotation rate of the drum, in revolutions per minute, at which resonance occurs.

2c
2 marks

During the spin cycle, the drum accelerates from rest to a maximum rotation rate of 1200 revolutions per minute.

The machine vibrates violently, but only for a short time while the drum is speeding up.

Suggest why the machine vibrates violently only for a short time.

2d
2 marks

The spring mounts are replaced with mounts containing oil. This increases the damping of the oscillations.

Explain the effect of this change on the amplitude of the oscillations at resonance.