Physics made easy by Dangerous Earth presenter Dr Helen Czerski

Find out how a raisin can explain the sinking of Titanic, and why an egg yolk is like the Hubble telescope


The best thing about physics is that it’s all about patterns, and the same patterns pop up in lots of different places. That means you can explain some really important situations using surprisingly mundane bits of everyday life. Here’s a taster.


Why is a raisin like the Titanic?

Put some raisins in a newly opened bottle of fizzy lemonade and watch them sink to the bottom because they are more dense than water. But their wrinkly surface is the perfect place for bubbles to grow, and so each raisin grows itself a life jacket of bubbles until the whole thing is less dense than water. Then it floats up to the top, tumbles around until the bubbles are lost and falls back to the bottom of the bottle – just like
a lava lamp. It’ll keep going for about half an hour, and it’s surprisingly mesmerising!

The physics…

The Titanic was lost because it was structurally very similar to a raisin covered
in bubbles. The ship itself was more dense than water, and only floated because it had
huge watertight compartments
full of air. These made it less
dense than water overall, just like
the bubbles on the raisin. But when
the ship hit the iceberg, the effect was
the same as the bubbles popping at
the top of the bottle. Overall, the Titanic became more dense than water so it had
to sink. Oddly, the size of a raisin relative
to the height of a lemonade bottle is almost exactly the size of the Titanic relative to the depth of the water it sank in.

How does a cup of tea explain earthquakes?


Put a mug of tea on a flat surface and give it a (small) shove. You’ll see the tea start to slop from side to side. This is known as oscillation. Each mug has its own natural slopping rate – slower in a big mug; faster in a small one. It just so happens that when we walk our natural step rate matches the oscillation rate of tea in the average mug – this is why we often spill tea when we walk in a hurry.

The physics…

In September 1985, a huge earthquake shook Mexico City. Afterwards, it was noticeable that most of the buildings that had collapsed were between five and 20 storeys high. The shorter and taller ones were fine. It turned out that the earthquake had shaken the ground at about once every two seconds, synchronising exactly with the natural oscillation rate of the middle-sized buildings. Just like tea spilling as you walk faster, the middle-sized buildings responded by swaying more and more violently – until they fell down. 

Why is a hi-vis jacket like a £10 note?

On a dull winter’s day, everything on the road looks a bit grey and drab apart from cyclists wearing high-visibility jackets. They look like they’re glowing, and it’s because they are. Low, heavy clouds get in the way of visible light (the colours of the rainbow) so most things look quite dull. But more of the light beyond the rainbow – ultraviolet rays – can get through. Fluorescent fabric takes the light we can’t see and turns it into visible light that we can – the bright glow of those jackets. 

The physics…

A cashier will quite often hold a banknote underneath a dim light. When this ultraviolet light touches fluorescent dyes on the banknote, those dyes convert this invisible light into bright red and green that we can see, and suddenly numbers are visible on the note that weren’t before – a good indication that the note is genuine.

How is an egg yolk like the Hubble telescope?


A raw egg and a boiled egg, both cold and still in their shells, look identical. How can you tell them apart? The trick is to set them spinning on their sides, let them spin for a little while, and then just touch your finger on the top of each one for a couple of seconds. The boiled egg will stop completely. But the raw egg will start spinning again when you remove your finger because you only stopped the shell and not the liquid. It’s a law of physics that once something is spinning, it will keep spinning until you stop it, so the egg’s innards give it away.

The physics… 

The Hubble gives us detailed pictures of incredibly distant stars. But how does something that is floating in free space stay on course? Hubble has gyroscopes inside that are basically giant versions of a spinning egg. Once they’re spinning, they’ll keep spinning about the same axis because there’s nothing in space to change the spin. This gives the telescope fixed internal pointers that always follow exactly the same direction. That’s how it can orient itself so precisely


Dangerous Earth is on tonight at 8pm on BBC4