Level 3 Physics

91523 (3.3): Demonstrate understanding of wave systems

Credits: Four

Check that you have completed ALL parts of the box at the top of this page.

Check that you have been supplied with the resource booklet for Level 3 Physics.

You should answer ALL parts of ALL questions in this booklet.

If you need more room for any answer, use the space provided at the back of this booklet.

Check that this booklet has pages 2–9 in the correct order and that none of these pages is blank.

YOU MUST HAND THIS BOOKLET TO YOUR TEACHER AT THE END OF THE ALLOTTED TIME.

OVERALL LEVEL OF PERFORMANCE

© New Zealand Qualifications Authority, 2012All rights reserved. No part of this publication may be reproduced by any means without the prior permission of the New Zealand Qualifications Authority.

SAMPLE PAPER

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School:

I give my permission for my answers to be placed on the TKI and NZQA websites as examples of student work, knowing that my name will not be used with the answer.

Signed:

Assessor’s use only

You are advised to spend 60 minutes answering the questions in this booklet.

QUESTION ONE: SHAKING THE STRING

Diagram 1

An oscillator shakes a string with a steady vertical oscillation of 5.0 Hz. A wave travels along the rope as shown in diagram 1 on the left.

(a) Explain why a standing wave forms. In your answer, you should explain:

what happens to the travelling wave after it reaches the fixed end

how the standing wave builds up from the fixed end

the standing wave that forms in the string

the general conditions for a standing wave to occur in a string.

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(b) The frequency of the oscillator is changed by a student to make the string oscillate at the 3rd harmonic. Describe (with a sketch or otherwise) the shape of the wave they will see, and calculate the frequency the student must use. You should use the information from diagram 1 for your calculation. Note: You should assume that the wave speed does not change when the frequency is adjusted and that the length of the string remains the same.

(c) When the 3rd harmonic is vibrating, the student increases the tension of the string so that the wave speed becomes 1.05 times faster; the student does not change the frequency. Predict and explain what the student will observe.

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QUESTION TWO: SOUND WAVE INTERFERENCE

Diagram 2

A teacher demonstrates interference of waves by connecting two speakers to a signal generator. The signal generator produces a single frequency. The demonstration is set up outdoors.

Students move along a line that is 3 metres from the speakers. They hear a change in volume as they move along the line. Two microphones are used to identify the points of maximum and minimum loudness. A microphone is placed at a maximum position, A, and at an adjacent minimum position, B (see diagram 3).

The loudspeakers are 0.52 m apart. The microphones are placed 3.0 m from the loudspeakers and the frequency is set to 1.30 kHz. The speed of sound is 320 m s–1.

(a) By calculating the wavelength of the sound waves (or otherwise), determine the distance between position A and position B.

Diagram 3

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(b) Discuss what the students hear as they walk at a speed of approximately 0.8 m s–1 along the line (as shown in diagram 2). In your answer, you should explain:

the cause of the loud and quiet positions

how the intensity will vary with time as they walk along the line

why the demonstration is not as effective in a typical classroom.

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QUESTION THREE: DOPPLER DUCK

Diagram 4

A teacher wants to make a video that demonstrates the Doppler effect. He uses a rubber duck to make circular ripples on a pond of still water. The teacher moves the duck up and down. Circular ripples spread out on the pond, as shown in the images above from two video clips. In the right-hand image above, the teacher was moving the duck with a horizontal velocity in addition to its vertical oscillation.

a) Identify the movement of the duck in each picture and explain how you can deduce the horizontal motion of the duck from the pattern of ripples.

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b) Careful analysis of the second video clip shows that the ripples are travelling at 0.400 m s–1, the teacher’s hand moves up and down 19 times over 5.00 s, and the wavelength of the ripples behind the duck is 0.135 m.

Use the information given to calculate:

(i) The frequency at which the ripples move a small twig floating directly behind the duck

(ii) The horizontal speed of the duck.

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Extra space if required. Clearly number the question (if applicable)

Questionnumber

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Assessor’s use onlyExtra space if required.

Clearly number the question (if applicable)

Questionnumber

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