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A shake handed from one bit of stuff to the next, what happens where there is no next bit, and how fast it goes.
Paper packet. Every task here also exists on screen, where it is checked automatically; answers written on paper are not assessed by Nydus. When you are back at a device, enter your answers there.
You will decide whether a sound can reach an ear by asking what lies between them, explain why a bell in a jar with the air pumped out can be seen ringing and not heard, and use the speed of sound to find how far away a storm or a cliff is.
You know that energy is passed along by a path, and that a push is something one thing does to another. Sound is both of those at once: a shake, passed on by pushes, from one bit of stuff to the next bit of stuff beside it.
You also know how to work out a distance from a speed and a time. Sound has a speed, so you can use that to find out how far away a thunderstorm is.
| Term | What it means |
|---|---|
| Vibration | A fast shaking back and forth. |
| Medium | The stuff a sound travels through: air, water, wood, steel. |
| Vacuum | A space with nothing in it at all, not even air. |
| Speed of sound | How far a sound travels each second: about 340 m/s in air. |
| Echo | A sound that bounces off something and comes back. |
| Loudness | How big the shake is; it changes how loud, not whether it arrives. |
| Pitch | How high or low a sound is, set by how fast the shaking is. |
An electric bell hangs on a thread inside a thick glass jar, ringing. A pump then takes the air out, a little at a time.
Pick one before you look.
| Air in the jar | What you hear | What you see |
|---|---|---|
| all of it | the bell, clearly | the hammer hitting it |
| about half | the bell, faintly | the hammer hitting it |
| almost none | nothing at all | the hammer hitting it |
| let back in | the bell, clearly | the hammer hitting it |
Three things to settle. Which prediction do the readings support? What would still be a possible explanation if the third column had gone blank? And why take the fourth row at all, when the answer was clear by the third?
Something that makes a sound is shaking: a bell's rim, a guitar string, your voice box. The shaking pushes on whatever is touching it, that pushes on the next bit along, and the shake travels outward from one bit of stuff to the next until some of it reaches an ear.
So sound needs a medium — something to travel through. Air is the usual one, but it is not special:
And in a vacuum there is nothing to hand the shake on to. The bell can shake all it likes; nothing arrives.
Handing the shake along takes time, so sound travels at a speed: about $340$ meters every second in air.
Another way: action
Stand a line of dominoes and push the first. The push travels the length of the line, but only because there is a next domino every time. Take three out of the middle and the push stops dead at the gap.
Another way: steps
Which prediction survives. Ana's. The ringing faded as the air went and stopped when the air was gone.
What the third column rules out. The hammer never stopped hitting the bell, so the bell stopped ringing was never available as an explanation. A column you do not need in the answer can still be the column that makes the answer trustworthy.
Why the fourth row. It puts the one thing that changed back and gets the old result back. Without it, somebody can always say the glass cooled or the bell wore out. Letting the air back in answers all of those at once.
And notice what you could do throughout: see the bell. Light crossed the empty jar perfectly well. Light does not need a medium and sound does, which is the one real difference between them at this stage.
Sound travels at different speeds through different stuff. The closer together and the more tightly joined the bits are, the faster they pass the shake on.
| Medium | Speed of sound | Time to cross 1 km |
|---|---|---|
| air | about 340 m/s | about 3 s |
| water | about 1500 m/s | under 1 s |
| steel | about 5000 m/s | about 0.2 s |
That is why an ear on a steel rail hears a train before the sound arrives through the air: the shake in the steel gets there about fifteen times as fast. It is also why whales can hear each other across great distances: water carries sound faster and further than air does.
Lightning and thunder happen at the same moment. You see the flash almost instantly, because light is fantastically fast: it could go around the Earth seven times in a second. The thunder is a sound, so it crawls along at about $340$ m/s.
That gap lets you measure how far away a storm is. Count the seconds between the flash and the thunder, then multiply by $340$ to get meters. A quicker rule: every three seconds is about one kilometer, or every five seconds is about one mile.
If the count is shrinking from flash to flash, the storm is coming closer. The National Weather Service says that if you can hear thunder at all, you are close enough to be struck, and you should go indoors.
When a sound hits a hard wall or a cliff, much of it bounces back. That returning sound is an echo. Because the sound has to travel to the wall and back again, the time you wait for the echo covers twice the distance to the wall.
Suppose you shout at a cliff and the echo comes back after $2$ seconds. The sound traveled $340 \times 2 = 680$ m in all, there and back, so the cliff is half of that: $340$ m away.
Bats and dolphins use echoes to find their way. A bat sends out a squeak too high for us to hear and listens for the echo off a moth. The shorter the wait, the closer the moth. Ships do the same thing with sound pulses in water to measure how deep the sea is, which is called sonar.
Two things about a sound can change without changing whether it arrives.
Loudness is how big the shake is. Hit a drum harder and its skin moves further back and forth, so the shake is bigger and the sound is louder. A louder sound still needs a medium; a bigger shake cannot cross a gap with nothing in it.
Pitch is how fast the shake is. A short, tight guitar string shakes back and forth very fast and sounds high. A long, loose one shakes slowly and sounds low. Pitch does not change the speed the sound travels at: a high note and a low note played together reach the back of a concert hall at the same moment.
Neither of these is the lesson's question. The lesson's question is only ever is there anything to carry it?
Humpback whales sing long, low songs that can travel for many kilometers through the sea. Water carries sound about four times as fast as air, and it carries low sounds a very long way before they fade.
Scientists from the National Oceanic and Atmospheric Administration listen with underwater microphones called hydrophones. If a song reaches one hydrophone $2$ seconds before it reaches another, the whale is about $1500 \times 2 = 3000$ m closer to the first. Using several hydrophones, they can work out where the whale is without ever seeing it.
The same physics explains why ship noise worries scientists. The shake from a ship's engine also travels far through the water and can drown out the whales' songs, which they use to find each other.
When astronauts walked on the Moon in 1969, they could not hear each other speak through the empty space between their helmets. The Moon has almost no air, so there is nothing to pass a shake from one helmet to the other.
Instead, each spacesuit had a radio. Radio travels as a kind of light, which needs no medium, so it crosses empty space easily. The astronauts' voices shook the air inside their own helmets, a microphone turned that into a radio signal, and a speaker in the other helmet turned it back into a shake of the air inside it.
If two astronauts touched their helmets together, though, they could hear each other faintly without the radio. The solid helmets pressed together carried the shake from one to the other: the rule once again, not an exception to it.
Most people already know you cannot hear anything in space, and remember it as a fact about space. Then they are surprised that a bell in a jar goes silent on a laboratory bench in an ordinary room.
The rule has nothing to do with space. It is: no stuff, no sound. Space happens to be a place with almost no stuff in it, and so is the inside of that jar.
Two smaller mistakes follow the same pattern. Solids block sound — they do not; they carry it, often better than air, and a door only muffles a knock rather than stopping it. And loud enough sound would get through — it would not. Loudness is how big the shake is, and no shake at all gets across a gap with nothing in it.
A last slip is thinking sound and light arrive together. They start together, but light wins every race by a long way, which is why the flash always comes first.
Find the shaking thing.
$\text{a train's wheels on the track}$
They shake the rail and the air.
Name the two paths.
$\text{through the air, and through the steel}$
Both carry the sound.
Compare the speeds.
$340\ \text{m/s against } 5000\ \text{m/s}$
Steel passes the shake on far faster.
Find the times over 2 km.
$2000 \div 5000 = 0.4\ \text{s}, \quad 2000 \div 340 \approx 5.9\ \text{s}$
Distance divided by speed.
Say what the listener hears.
$\text{the rail first, the air later}$
Solids carry sound well.
Read the first count.
$9\ \text{s between flash and thunder}$
The sound's travel time.
Find the first distance.
$340 \times 9 = 3060\ \text{m}$
Speed times time.
Read the next count.
$6\ \text{s}$
A few minutes later.
Find the next distance.
$340 \times 6 = 2040\ \text{m}$
Speed times time again.
Compare the two.
$3060 - 2040 = 1020\ \text{m closer}$
The count is shrinking.
Decide what to do.
$\text{go indoors}$
If you can hear thunder, you are close enough to be struck.
Name the medium.
$\text{seawater}$
The pulse travels down through the water.
Recall the speed in water.
$1500\ \text{m/s}$
Faster than in air.
Read the echo time.
$4\ \text{s from pulse to echo}$
Down to the seabed and back.
Find the whole trip.
$1500 \times 4 = 6000\ \text{m}$
Speed times time.
Notice it goes both ways.
$\text{down and back up}$
An echo covers the distance twice.
Halve it for the depth.
$6000 \div 2 = 3000\ \text{m}$
The seabed is three kilometers down.
Check it makes sense.
$3\ \text{km} < 11\ \text{km, the deepest ocean}$
A real depth for the open ocean.
Name what lies between.
$\text{the water}$
Ask what is between the shout and your ear.
Decide if it can carry sound.
Compare its speed.
Think about a rock splitting on the Moon, watched from a spacecraft. Between the sound and your ear there is nothing at all. Do you hear anything?
Complete the worked solution: a hiker shouts toward a cliff and hears the echo $4$ seconds later. Sound travels at $340$ m/s in air. How far away is the cliff?
Find the whole trip.
$340 \times 4 =$ a m
Speed times time gives the distance the sound traveled.
Notice the trip goes both ways.
$\text{to the cliff and back}$
An echo is the shout bouncing off the cliff.
Halve it for the cliff.
$\text{whole trip} \div 2 =$ b m
The cliff is half the whole trip away.
Match each sound to the stuff that carries it to the ear.
| the air | the water | the steel | nothing at all, so nothing is heard | |
|---|---|---|---|---|
| a bell ringing on a table in a room | ||||
| a swimmer tapping two stones together underwater | ||||
| an ear pressed against a long steel rail | ||||
| a bell ringing inside a jar with the air pumped out |
Put the five steps of the bell-in-a-jar demonstration in the order they happen.
Number the steps in order (write the number in the box):
Four explanations of why the bell in the emptied jar cannot be heard. Mark the one that is right.
This task has no paper form; do it on a device.
For each of these, write yes or no: does the sound reach the ear?
| does the sound reach the ear? (yes or no) | |
|---|---|
| a knock on the far side of a wooden door | |
| a bell ringing inside a jar with the air pumped out | |
| a shout through a closed wooden door |
The bell inside this jar is still swinging, and the pump has taken all the air out. Click the part of the picture where there is now nothing for the sound to travel through.
This task has no paper form; do it on a device.
During a storm, a student sees a flash of lightning and counts $12$ seconds before the thunder arrives. Sound travels through air at about $340$ meters each second. How far away was the lightning, in meters?
Answer: unit: m / cm / km / mm
In a movie, two spaceships fight outside a space station and the explosions are deafening. An astronaut watching from a window nearby would in fact hear what?
Lesson test: one question per skill, one attempt each, no hints. Your answers are checked when you submit.
For each of these, write yes or no: does the sound reach the ear?
| does the sound reach the ear? (yes or no) | |
|---|---|
| an ear pressed against a long steel rail | |
| a rock splitting on the Moon, watched from a spacecraft | |
| a shout through a closed wooden door |
You can say what carries a sound in any situation, and when nothing does. Tell someone why a knock comes through a closed wooden door.
17. Your turn: why can you hear a friend shouting at you from the far side of a swimming pool, underwater?, step 2
$\text{yes: water passes a shake along}$
Just as air does.
17. Your turn: why can you hear a friend shouting at you from the far side of a swimming pool, underwater?, step 3
$1500\ \text{m/s, faster than air}$
It even arrives sooner than it would through air.