Resonance is vibrating along: if a vibration comes in at exactly the natural frequency of an object, every vibration adds to the previous one and the amplitude keeps growing. At any other frequency almost nothing happens, however hard you push. So it is about timing and not about force.
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I sing as loud as I can at a glass, and nothing happens. That is not about how loud I sing. In this video you see why some things suddenly vibrate along and most do not.
Everything that can vibrate has a natural frequency: the rate at which it vibrates by itself when you tap it. If a vibration comes in from outside at exactly that frequency, the object goes along and the amplitude keeps growing. That is resonance. If a vibration comes in at another rate, almost nothing happens, however hard that vibration is. You see it on a swing, where small pushes at the right moment bring you much higher than one hard push, and you see why a glass still almost never breaks: there is damping as well, so you need the right tone and enough volume. And it is not only about sound by a long way, because a footbridge and a stand do exactly the same.
For upper secondary physics, in the chapter on sound. Halfway there is a question and a silence: pause it there and think for yourself first.
In this video
The full explanation, in writing
The problem
0:00I sing as loud as I can at this glass. As loud as I can. And nothing happens. It is not about how loud I sing.
Every thing has a rate
0:15Tap a glass. You hear a tone. Tap again, and you hear exactly the same tone. Take another glass and it gives a different one, but that one too is the same every time. That is no accident. All that can vibrate has a rate at which it does so by itself.
0:30Tap it, it rings out at that one rate and at no other. That rate we call the natural frequency of that object. You also meet it as the resonant frequency.
What resonance is
0:45And now the nice part. If a vibration comes in from outside that has exactly that natural frequency, the object goes along. It starts vibrating at the same frequency as the source. That is called resonance. If a vibration comes in at a different frequency, almost nothing happens.
1:04The object gets a push from one side while it just wants to go the other way, and those two cancel each other out. So it is not about how much vibration comes in. It is about whether the frequency is right. You push someone on a swing. What works better: one very hard push, or a small push each time?
Small pushes at the right moment
1:29Anyone who has ever pushed knows the answer already. Small pushes at the right moment bring you much higher than one enormous push. The frequency of a swing you can work out. Swings with ropes of over two metres take about three seconds for one there and back, and that is zero point three three vibrations per second.
1:51If you push at exactly that frequency, every push arrives while the swing is already going that way. Every push then adds to what was already there. If you get three per second, you have turned one divided by three round. And there sat my mistake.
2:11I sang loudly, but not at the right frequency. Not at the resonant frequency.
Why it still almost never works
2:18Back to the glass. If you sing exactly the tone of that glass, it vibrates along, and every vibration makes the amplitude bigger. Long enough, and the rim can no longer follow and it breaks. Only there is something working the other way. Vibrating objects also lose energy, to the air and to themselves.
2:40That is called damping, and it is always there. So the amplitude only grows as long as you put more in than the damping takes out. So you need two things at once, and not one. Exactly the right tone, and enough volume. Over a hundred decibels, to be precise.
3:00That is why those videos there is always an amplifier next to it, and why I cannot do it at the table.
The same effect, but useful
3:08Resonance is not only something that breaks things. It is also the reason an instrument can be heard. One vibrating stretched string sets far too little air moving on its own. You hear almost nothing of it. Put that string on a hollow box, and the air in that box vibrates along, at the same frequency as the string.
3:30This makes the sound louder. Put something that vibrates on a table top and you hear the same: the same vibration, suddenly much louder, without you adding any energy.
Where it goes wrong
3:43This is not only about sound by a long way. Footbridges sway by themselves at about one to three times per second. Marching soldiers take about two steps per second. Those two can meet each other, and then every step adds to the previous one, just as with the swing.
4:02That is why there is a rule that soldiers break step on a bridge. Out of step the steps come at random and cancel each other out. The same holds for a stand where the public jumps in time. It is not their weight that is the problem, because that stands on it when still as well.
4:23It is the resonant frequency. Three things to remember. One: everything that can vibrate has its own natural frequency. Two: if a vibration comes in at exactly that resonant frequency, the thing vibrates along at the same frequency and the amplitude keeps growing. That is resonance.
4:45Three: it is about timing and not about force. Pushing hard at the wrong moment does nothing, and small pushes at the right moment do everything. Two questions. First question: a washing machine makes the whole floor vibrate, but only at one particular spin speed.
5:02Why there, and not at a higher speed? Second question: someone says you can also break a glass by shouting very loudly at it, whatever the tone. Is that right? The first: at that one speed the frequency of the drum coincides with the resonant frequency of the floor.
5:25Then every turn adds to the previous one. If the drum turns faster, the frequency no longer fits and little happens. And the second: no. Shouting does give a lot of volume, but it is a messy sound with all sorts mixed in. Only a small part sits at the tone of the glass, and the rest does not help.
5:49You need both: hitting the resonant frequency and enough volume. You now know why some things suddenly vibrate along and most do not, and that you must hit the resonant frequency and not the volume. Why two instruments that play the same tone still sound different, there is a separate video about that on the channel.
Frequently asked questions
Why does a glass not break if you shout at it very loudly?
Because shouting is a messy sound with all sorts of frequencies mixed in, so only a small part of it sits at the tone of the glass. You need both: hitting the resonant frequency and enough volume, over a hundred decibels.
What is the difference between natural frequency and resonant frequency?
They are two names for the same thing: the rate at which an object vibrates by itself when you tap it. It is called the resonant frequency because it is the frequency at which an incoming vibration makes the object resonate.
Why do soldiers have to break step on a bridge?
Because a footbridge sways by itself at about one to three times per second and a marching column takes about two steps per second. Those two can coincide, and then every step adds to the previous one, just as with a swing. Out of step the steps come at random and cancel out.
What is damping?
The energy a vibrating object loses, to the air and to itself, and it is always there. The amplitude only grows as long as you put more in than the damping takes out. That is why resonance needs enough volume and not just the right tone.
Why does a string sound louder on a hollow box?
Because the air in that box vibrates along at the same frequency as the string. A string on its own sets far too little air moving, so you hear almost nothing of it. This is resonance being useful instead of destructive.
Pushing hard at the wrong moment does nothing. Small pushes at the right moment do everything.
Physics with Thomas makes physics explainers for secondary school. No words on screen, so what you see works in any language.
About me
My name is Thomas Schuurmans, and I have a passion for physics and for understanding why things work the way they do. What I want is to get young people asking “why is that?” and “how does that work?” more often, and to hand them the tools of physics in a way that is simple and that you can see.
I graduated in applied physics at Delft University of Technology in 2003. After that I spent more than twenty years outside education: first at TNO, the Dutch applied research institute, then at a design agency, and eventually founding Proportion Global, through which I work on innovation questions in Africa, Latin America and South Asia. That work resembles physics more than you would expect: don't start from a solution, first understand what is going on, try something, be wrong, and look again.
Since 2026 I have been teaching physics to both lower and upper secondary classes, and I started a master's at the University of Amsterdam for my full teaching qualification. That is where I learned that a secondary school pupil's real attention for new material lasts about seven minutes. And my own weakness happens to be telling too many side stories.
That is where the idea came from: videos that explain one topic sharply and visually, inside those seven minutes. I make them for my own pupils. Then I publish them, because good explanation should be within reach of anyone who needs it, wherever you live and whatever language you think and speak in.