About this blog

Physics can be difficult to learn, but this blog aims to help you get into physics by connecting your GCSE physics lessons with things you see in the world around you.

Tuesday, 6 March 2012

A Voice That Could Break Glass

My sister. No, she doesn’t have a voice that could break glass. But she does have an inquiring mind combined with an idea that Physics is probably just a conspiracy. I’ve no idea why she suddenly decided to text me early on Sunday morning asking if you could break glass with sound, but she did. She was so suspicious of my texted reply that I needed to elaborate - so here is my answer.

Let’s start with sound waves. We know that of the two types of waves, transverse and longitudinal, that sound waves are longitudinal. That means that the sound source vibrates backwards and forwards and moves the air molecules accordingly.


Under normal circumstances though, these sound vibrations can’t really move much apart from your ear drum, this being how you hear the sounds. There are a number of things you can measure that tells you more about the waves – the amplitude, the wavelength and the most important one here – the frequency. The frequency means how many waves per second there are and this is measured in Hertz. The higher the frequency, the higher the pitch of the noise that you hear.

So, we know that for most frequencies, sound cannot break glass. We know this from our everyday lives – the glass we drink out of, the glass in the windows, they don’t just spontaneously shatter on a regular basis, not even when someone really annoying talks right by them.

Ok, a small aside. You know how when you’re swinging on a swing, if you get your legs helping you just right then you’ll keep getting higher and higher, but if you get your legs wrong it’s impossible? That’s all about working with the frequency that the swing naturally wants to swing at and not trying to make it go faster or slower.

The same is true with sound waves and breaking glass. Once you get the right frequency, what starts off as teeny tiny vibrations of the air next to the glass and then the glass itself, if you can get the waves pushing at just the right time, the glass will start to vibrate, then ripple more and more and more. This is called the resonating frequency.

Obviously, glass is a brittle material, but when you get it at its resonating frequency it almost looks like it’s acting like water. Well, that’s until it smashes! After all, it’s still glass.

Fancy checking this out? Search youtube for "resonating sound breaking glass" and there are some great examples!

Friday, 17 February 2012

A Bit Dim

Light switches – have you ever noticed that some of them just let you switch the light on and off, whilst others give you the power to choose how bright you want the light to be? How they do this is pretty simple, so let’s look at it a bit more closely.

We’ve previously covered resistance here in electronic wires, with the contributing factors being 1) material type 2) wire length 3) cross-sectional area 4) temperature. So let’s think which of these we can change of a varying basis depending on what brightness we want.

It’s a pretty costly solution to have different wires made from different materials running all round your house just in case you want to dim the lights, and it would be hard to get a continuous scale too. Likewise cross sectional area would require a huge amount of extra wiring. That leaves us with temperature and length.

Whilst I like the idea of changing the brightness of a bulb by changing the temperature, it seems a little impractical! So that leaves us with length. You need quite a lot of wire to change the resistance so dramatically that you will notice it in the brightness of your bulb, but the dimmer switch is usually just a rotating knob. The wire is coiled up on itself so that rather than being able to choose from any point along its length, you make contact with one spot along the coil.

That way you get a lot more change in resistance with only small turns of the dial -  meaning that rather than a huge distance moved you can just use a rotating switch. This type of resistor is called a variable resistor and its symbol is:

It’s used for other applications too, can you think of any?

Tuesday, 7 February 2012

Precision Counts

Watching the TV the other night, I was confounded by an advert that promised “the search for fast”. And I thought, shouldn’t that be “the search for speed”? Now, I know that arguing with the TV is pointless, and definitely always ends up with you looking a bit mental and nothing resolved, but this one really stuck in my brain.

What is fast exactly? As a scientist, it’s a loose term you can use to describe how someone or something moves, eg that car is moving fast. But it doesn’t really tell you anything, since it’s relative. If the car were overtaken by a train, you might say that the car is still fast but the train is faster. But that’s not really the whole of the story.

When it comes to science, it’s not just numbers that you need to worry about – words are important too. They can tell you a lot about the problem that you’re trying to solve so pay attention! I’ve spoken several times about the difference between speed and velocity, with speed being a directionless way of measuring how fast something is going, whilst velocity is also about direction.

What that means in practical terms is that if you imagine a car going around a race track at a steady speed of 50 mph, the velocity will be changing as the track twists and turns. This can be a confusing concept to start with, but once you get to grips with this and other important distinctions in the language of science, you’ll start to see things in a new light.

Start by thinking about what the difference is between weight and mass for example (hint, one of them changes with gravity and one doesn’t) or between force and weight.

Ok, so becoming a bit picky and precise about words might lead you to rant about an advert and its search for “fast”. What is that? And how do you search for it?? But at least you’ll be thinking critically about what words to use to accurately describe things, and that will make me glad.

Tuesday, 17 January 2012

Absolute Zero

You might think that it’s been pretty cold the past few days, especially compared to the warm wintry days we’ve been experiencing recently, but what’s the coldest weather you’ve ever felt?

I’ve probably been in around -10°C, possibly a little colder, although not usually being in the habit of carrying a thermometer with me I can’t say for absolute certain. Most of us, unless we visit the Arctic or climb Mt Everest won’t get the chance to be really really cold, like -40°C.

And yet, there’s still colder, so cold that there’s another temperature scale to make it easier to understand and write about. In the UK we usually use Centigrade to measure temperature, although the older generation often prefer Fahrenheit. These have a complicated relationship and it’s hard to translate one into the other in your head.

The Kelvin scale (symbol °K) starts at absolute zero, the coldest possible temperature, at which even atoms stop their constant vibrations. It’s the same as the Centigrade scale but starts at -273°C. We’ve (well, not me personally, scientists ) managed to measure to within 0.000000000000001 degrees of absolute zero, which is pretty close in my book!

Friday, 13 January 2012

Skating on Water

Dr Em’s fun fact for Friday 13th: whenever you go ice skating, you actually skate on water.


Let’s break this down. Remember how your mum used to tell you that ice skates could cut people’s fingers off? That might be just a scare tactic or an excuse but there is some logic behind it. It’s all to do with the fact that ice skates have a very very small area that’s in contact with the ice. That means a very high pressure.

If we say that the ice-skate blade is 25cm long and 1mm wide, we can work out the total area in contact with the ice.

A = 0.25 x 0.001 = 0.00025 m-squared

Pressure is Force divided by Area so presuming that you weigh 60kg let’s find out the pressure. Assume gravity is 10 m/s/s

F = weight x gravity = 60 x 10 = 600N


P = 600 / 0.00025 = 2400000 Pa = 2.4 MPa

That’s quite a large amount of pressure, certainly enough to melt the localised patch of ice underneath your blades. Of course, it freezes over again more or less as soon as you’ve passed, but the fact remains that you ice-skate on water.

As for fingers, I think we need some volunteers to see whether 2.4 MPa is sufficient to remove one of those…any takers? On second thoughts, that sounds too much like Biology for me.

Tuesday, 10 January 2012

Rock, Paper..Water?

As I was playing rock, paper, scissors the other day, I thought how stupid it was that paper beats rock. I mean, sure, it covers it…but that’s about all. So I was wondering if I could come up with a better trio to play the game with. And because all thoughts in my brain lead to science I thought I should share some of the match-ups I considered

Rock v Water

Seems simple, right? Rock goes in the water, and you could argue that the rock wins as it stabbed through the water, or that the water wins since it absorbed the rock. I’m thinking more technical though. When water freezes it makes ice, which we know takes up more volume than as a liquid (test this by filling a water bottle full and putting in the freezer, it’ll be all swollen). In cold places, water creeps down the holes in rocks, then when it freezes, pushes the rock apart, over time splitting huge bits of rock off. So, water most definitely beats rock then.

Paper v Chalk

Chalk writes on paper, paper gets written on, seems fairly straightforward that chalk wins. But don’t be so hasty! Have you seen those card throwers? Go on – look them up on youtube, I’ll wait. They have the ability to cut, pierce and slice with nothing but a playing card or business card and the right throwing technique. It’s all about the pressure that you can create with the card edge, and pressure comes from force. A flying card has plenty of forwards thrust thanks to the thrower, and combine that with the tiny area of the card edge and you can see how easy it would be to get the card to pierce something…like a piece of chalk. So, paper could win!

Water v Paper

If this was a game played with hands I wouldn’t like to try and guess how water would be done, but thankfully that’s not part of the problem I’m worrying about here! Paper goes in the water and gets soggy seems like too much of an obvious answer here. Let’s imagine we’ve got a piece of paper that won’t get soaked too easily, like a playing card again. Take a glass of water, put the card over the top and carefully turn upside-down. Now lift up the glass… Careful! You’ll find that the paper has trapped the water, thanks to the way that the water forms a seal and won’t let more air in, effectively creating a semi-vacuum that holds the water up.

So it seems that rather than creating a new game that actually works, I’ve just been busy defending poor paper. Not just “wins by covering rock” but much more awesome than that, thanks to a few little bits of physics. Paper cuts rock (well, chalk) and paper traps water. Go paper! I’ll always choose you now. Just beware the scissors, there’s nothing that can save you there.

Thursday, 5 January 2012

Water Water Everywhere

Rain and wind, that’s all we seem to get at the moment, morning, noon and night. Wouldn’t it be nice if we could actually benefit from the elements rather than just complaining about them? This is where renewable energy generation comes in. Unlike solar power, wind and rain are more frequently abundant when we need the most power, when we’re cold during the winter.

Wind turbines and hydroelectric power (the posh name for electricity from water) stations have their origins centuries ago when windmills and watermills were both used to process grain into flour, which is where the “mill” part of the name comes from. Turning your energy source into mechanical motion is a nice and efficient way of using the energy, but there’s no way of storing it or sending it elsewhere if you have too much to use.

So turning the power of wind and water into electricity makes it much easier to share, although it’s still pretty tricky to store. Good job it seems to be windy and rainy all the time then! But how do we go about getting electricity from the wind or the rain? It’s not like you can just hang your plug out of the window and get it that way!

Remember how we learned about getting electricity from fossil fuels (previously)? Well this works just the same, but rather than burning the fossil fuel to make steam to turn the turbines, the water or the air turns them directly.

This can be problematic though, because whilst you can accurately control the steam pressure when you’re burning the fuels, there are no such controls on the elements. It’s usually a trade-off between getting some power out when there’s not such a high flow, and making sure that you don’t have the turbines running too fast when the winds are howling or the water is racing.

Wind turbines are often geared towards moderate winds, so you’ll notice that sometimes when it’s too windy that the turbines are switched off. This is partly for health and safety but also because they just can’t cope when the gusts are really racing, but they can generate electricity in all but the most still conditions.

Water is easier to control, because you can build dams, but that can get costly and people don’t tend to like it when you flood the valley that they live in! There are also many more types of water powered electricity generators, with things like power from the tides, underwater currents in the sea, and storm water becoming increasingly likely to be a big player in the future of renewable energy.