Understanding Resonance

Time for a physics lesson!

Resonance is highly important in day to day life. Everything has a ‘natural frequency’, which is fundamentally the frequency which an object oscillates at when set into motion, or to put it simply, if you make an object like a swing move and leave it, the time it takes to do one swing (or oscillation, if you pushed it from it being still, one oscillation would be when it swung forward, then back and when it reached the middle again, that would be a full oscillation) + some simple maths gets you its natural frequency.

Example – Newtons Cradle, but with just one ball – You make the ball move, measure the time it takes to do one oscialltion (one backwards and one forwards swing). To get the frequency, do 1/the time it takes to do one oscillation in seconds. The units are Hz for Frequency. So if it took half a second to do one full swing, 1/0.5 = 2Hz . So the natural frequency would be about 2Hz.

Now why is natural frequency important when we are talking about resonance. Well, resonance occurs when an object is forced to move at its natural frequency. I’m not going to go into complicated and long winded although quite simple physics to explain why if an object is made to move at its natural frequency, it resonates, but its to do with adding and subtracting waves…maybe in another post – it needs lots of diagrams!

You might think its a bit pointless, but actually it saves lives. Remember when the millenium bridge was wobbling, that was because the wind was making the bridge move at its natural frequency.

One famous example was the Tacoma Narrows bridge in Washington, USA. One November morning, a high wind blew which was enough to make the bridge resonate. For something like concerete, the natural frequency is so little, its hardly noticeable, but when it resonates, boy do you realise it!

Resonance doesn’t just apply in things you would think of swinging. Have a look at this video of a British Army Chinook aircraft. The Chinook, if remaining stationary on the ground for two long, will vibrate at its natural frequency when the propellors start. What is happening is that it is coming close to resonating and the waves of the propellors and of the Chinook are adding up which starts to cause the aircraft damage.

Thats all for today folks. If you have any questions, feel free to ask, I’m on this regularly so any basic-ish physics stuff I am happy to explain or help with, maybe even some complicated stuff – to English kids wanting help, I did physics A-level so that is the extent of my knowledge so far 🙂


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