Pupil sheet · Sound waves
Sound needs a medium — questions
Name ______________________ Date ____________
Try every question before you use a nudge. Write in full sentences where you are asked to explain.
1. explain
As a teacher removes air from a bell jar, the hammer keeps hitting but the sound outside becomes fainter. Explain what each observation shows and what letting air back in should do.
Evidence about the source
Why sound fades
Air let back in
Nudge (only if you are stuck): Separate the vibrating source from the path carrying its sound.
2. identify / explain
Which of these can carry sound? Choose all that apply. Then explain why you left out any option.
Tick every correct statement.
- A perfect vacuum
- Helium gas
- A wooden table
- Sea water
Why you left out any option
Nudge (only if you are stuck): For each option, ask: are there particles there to pass the vibration on?
3. calculate
A firework bursts high above a park. A girl in the park hears the bang 1.5 s after she sees the burst. Calculate how far the firework is from her, using 340 m/s for the speed of sound and neglecting the time the light takes.
Distance: ______________ unit ________
Nudge (only if you are stuck): The light reaches her almost at once. Which journey does the sound make, and for how long?
4. calculate
A sound travels 680 m through air at 340 m/s. Calculate its travel time.
Travel time: ______________ unit ________
Nudge (only if you are stuck): Time = distance ÷ speed; 340 m/s means 340 metres in each second.
5. order / explain
Supplement. Use the table to order these media from slowest to fastest sound speed. Explain which would give the earliest arrival if the sound travels the same distance through each.
| Medium | State | Approximate speed of sound / m/s |
|---|---|---|
| Sea water | Liquid | 1530 |
| Aluminium | Solid | 6400 |
| Carbon dioxide | Gas | 260 |
Write the number of each step in order.
- Aluminium
- Carbon dioxide
- Sea water
- First: ______
- Second: ______
- Last: ______
Earliest arrival for the same distance
Nudge (only if you are stuck): Read the speed column, not the state column. For the same distance, does a greater speed give a shorter or a longer time?
6. explain
Huge explosions happen on the surface of the Sun. Telescopes on Earth record their light, but no one on Earth has ever heard them. Explain why.
Nudge (only if you are stuck): Compare what light and sound each need along the 150 million km between the Sun and Earth.
7. calculate
Thunder is heard 2.7 s after a lightning flash. Calculate the distance to the lightning in metres, using 340 m/s and neglecting the light travel time. Give your answer to 2 significant figures.
Distance: ______________ unit ________
Nudge (only if you are stuck): Check your answer: should it be more or less than 340 m, the distance sound covers in 1 s?
8. identify / calculate / explain
On sports day a timekeeper stands 250 m from the starter. She starts her stopwatch when she sees the smoke from the starting pistol and stops it when she hears the bang. Her five times are 0.74 s, 0.73 s, 0.75 s, 0.52 s and 0.74 s. (a) Identify the anomalous time and calculate the mean of the other four. (b) Calculate the speed of sound, giving your answer to 2 significant figures. (c) Explain whether your answer agrees with the speed of sound in air.
(a) Mean of the other four times: ______________ unit ________
(b) Speed of sound: ______________ unit ________
(a) The anomalous time; (c) does your speed agree with air?
Nudge (only if you are stuck): Leave out the time that does not fit the others. Then use speed = distance ÷ time.
9. state / explain
Supplement. The table gives approximate speeds of sound in three media. (a) State the general comparison of sound speeds in solids, liquids and gases. (b) Explain whether the data in the table agree with it. (c) Explain how sound passes through copper even though copper cannot flow.
| Medium | State | Approximate speed of sound / m/s |
|---|---|---|
| Helium | Gas | 1000 |
| Copper | Solid | 4700 |
| Ethanol | Liquid | 1160 |
(a) General comparison
(b) Do the data agree?
(c) How sound passes through copper
Nudge (only if you are stuck): Compare one pair of states at a time: the solid with the liquid, then the liquid with the gas.
10. explain
A display model has two sealed air-filled chambers separated by an ideal vacuum gap, with no solid route for vibration between them. A bell in one chamber keeps vibrating. Explain why a listener in the other cannot hear it directly across the gap, even though there is air near both the bell and the listener.
Nudge (only if you are stuck): Follow the whole route, including the space between the two chambers.
11. calculate / explain
A worker strikes a steel post that stands in the sea beside a jetty. The post sends sound into the air and into the water at the same moment. A listener on the shore is 1.19 km away through the air; a diver is 6.12 km away through the sea water. (a) Calculate the time the sound takes to reach the listener, using 340 m/s. (b) Supplement: sound travels at about 1530 m/s in sea water. Calculate the time the sound takes to reach the diver. (c) Supplement: a pupil says, “Sound is faster in water than in air, so the diver hears it first.” Explain why the pupil is incorrect.
(a) Time to the listener: ______________ unit ________
(b) Supplement: time to the diver: ______________ unit ________
(c) Supplement: why the pupil is incorrect
Nudge (only if you are stuck): Change kilometres to metres first. Is each part asking for a speed, a distance or a time?