How Dangerous is Your Holiday – 5. Swimming
Many holidays involve swimming – here are three activities that reveal some of the science involved.
Dick and Dom find out why it’s easier to float in salty water, how to keep dry underwater, and what makes a bellyflop.
1: Floating and sinking
Have you ever noticed that you float more when you’re swimming in the sea than in a swimming pool? Here’s a simple experiment to demonstrate why.
You will need:
- two containers, such as small bowls
- two same-sized raw eggs
Fill both containers with water. Now add several tablespoons of salt to one of them, and stir it round.
Now gently drop one egg into each of the containers. As you’ll see, the egg in the salty water floats, but the other one doesn’t.
When you add salt to the fresh water, it becomes denser. This means that if you had the exact same volume of fresh and salty water, the salty water would be heavier. The salty water is denser than the egg, so the egg floats on top. The other egg is denser than the fresh water it’s in, which is why it sinks. This explains why humans float more in the salty water of the sea than we do in a swimming pool.
2: Diving into water
What’s the difference between a dive and a bellyflop? Here’s an experiment that demonstrates what’s going on, and you don’t even have to leave your house to do it...
You will need:
- Two same-sized pieces of modelling clay
- A large bucket of water
Mould the first piece of modelling clay into a thin sausage shape with a neat pointy end. When you drop this one, pointed end first, into the water, it will probably make only a very small splash. This is like an expert diver entering the water. Now mould your second piece of clay into a flat pancake. Let it drop down, horizontally. You’ll see it slap the water, making that classic sound of a bellyflop. Yeowch!
This experiment is all about the surface area of the piece of clay (or diver) that enters the water first. Water has a 'skin' and an object has to separate that skin to dive in as well as pushing the water out of the way. If you dive in neatly, you only have to separate a small bit of skin and move a small area of water, but when you do a bellyflop the area is too large and the water can't separate quickly enough. So your stomach meets the flat of the water on impact.
3: Keeping Dry
Here’s a demonstration that shows how you can keep something dry, even when you submerge it in water.
You will need:
- A large clear plastic container – so that you can see what’s going on from the side
- A mug
- A glass roughly the same size as the mug
- A tissue
Fill the large container with water. Scrunch the tissue up, and push it into the top of the mug so that it will stay there and not drop out. Turn the mug upside down and submerge it in the water. What do you think will happen to the tissue? Now remove the mug keeping it upside down until it’s out of the water. Examine the tissue – it’s dry. Now repeat the above steps but using the glass instead of a mug. This time you can see exactly why the tissue remains dry.
At the start of the experiment, there isn’t just a tissue in the mug, it’s also full of air. This air pocket stays in the mug when you put it into the water, and doesn’t allow much water into the mug. So the tissue, which is trapped high up in the mug, stays dry. Try putting the mug with the tissue in the other way up – i.e. with the opening at the top – and you’ll see a very different result. The air in the mug is free to move out of the way, so the water floods in, and the tissue is soaked.