Science For Families: Floating
© and courtesy Nevit Dilman
How cool is your little Cartesian Diver? It’s amazing that you can make it float or sink with just a little squeeze of the bottle.
Knowing how things float and sink can be really useful, and there are loads of different ways to explore the idea. For example, using gas:
Sparkling drinks are fizzy because of a gas (carbon dioxide) that’s dissolved in the water. The gas forms little bubbles which rise to the top, where they burst. If you put some raisins in a glass of sparkling water and watch what happens.
The tiny little dents and indentations in the side of the raisin give the gas something to form a bubble around. When the raisin has enough bubbles it can float up to the surface. The bubbles then pop, the raisin sinks and the whole merry dance begins again. Try making an artificial raisin out of blu-tac. Remember it’s all the little tiny nooks and crannies that allow the bubbles to form. It’s surprisingly difficult to get it to dance like the raisin, but it is possible, so keep trying!
A similar thing happens when you put Mentos sweets in a bottle of Coke. The tiny imperfections on the surface of the Mento acts as sites for bubbles to form. Lots of bubbles. Lots of bubbles. What, you’ve never put Mentos in Coke? Go outside and do it right now!
Displacement
The key to being able to make things float is displacement. When a boat floats it shoves water out of the way – it displaces it. The displaced water weighs as much as the boat. But if the boat’s too heavy for its size it can’t displace enough water to balance out, and it sinks. This means you can build boats out of metal, as long as they’re large enough to displace enough water to offset their weight.
You can have a go at boat design using kitchen foil and pennies. Make a boat shape from the foil, and check it floats. Then load it up with pennies. How many could you get in your boat before it sank?
Try a redesign, and try again. Remember, you want your boat to take up as much space as possible, so that it displaces a lot of water.
Density
© Marie-Lan Nguyen / Wikimedia Commons / CC-BY 2.5
Another way to make your boat float is to make the water heavier.
Wait – what?
When you add salt to water and dissolve it, you are increasing the density of the water – it takes up the same space, but weighs more than it did. This is why it’s easier to float when you’re swimming in the sea than when you’re in a pool, the sea is saltwater. If you’re in the dead sea, which has an unusually high salt content, then you float very easily indeed.
You can test this out using a bowl of water and an egg. If you put an egg into a glass of tap water, it will sink. However, if you add a lot of salt and dissolve it in the water first, you can get the egg to float – a great trick to bamboozle your friends!
Goat on a Boat
Goat on a Boat
A goat? On a boat?!
What’s that got to do with science?
In fact, it’s very scientific when you know that the youngest of the Billy Goats Gruff was actually a naval architect (a boat designer). This little known twist in the old tale has inspired our Goat on a Boat workshop – where we really do float a goat on a boat!
The Workshop
Goat on a Boat has been created to help you explore the world around you, at around Key Stage 1. You’ll practice reasoning skills to develop fair experiments, and from your observations you’ll solve real problems. Real problems like:
Hmm. Perhaps not very real problems. But real enough for the youngest billy goat.
We provide you with the materials to build your raft, some test weights (washers)… and a goat. You could supply your own goat, but ours are carefully trained to be minimal bother in workshops. They’re also small and plastic.
You have to design, build and test a foil boat which is strong enough to float even the youngest goat safely across a river.
- What design will your boat be?
- How will you test your boat? How will you know which is the best design?
- How will you record your observations?
During the workshop, you’ll experience an awful lot of sinking and hopefully a bit of floating as well. You’ll explore which shapes of boat are better than others for keeping goats afloat.
By the end of the workshop you should have a really good idea of how to make a boat that can float. Hopefully, you’ll have made a firm recommendation to your intrepid goat, and will have floated it happily on your boat.
Parents – continue this at home
Nurturing your child’s natural curiosity will help them practice key skills. At home, you could explore the idea of floating and sinking by making simple boats out of aluminium foil (aluminium food trays are particularly good). Try making boats of different designs, and test them in the bath or sink. Use toys to test how much weight they can carry. Why are some designs better than others?
You could also explore whether some objects are buoyant – whether they float. Collect a bunch of things you don’t mind getting wet, and together, make predictions about what you think will float, and what will sink. Then test out your predictions, and try to explain why you got some right, and others wrong.
“Tell me about…” is a particularly useful phrase. It’s a less direct challenge than “Explain this…”. Science is more about what you can observe and describe than what you already know.
Goats aren’t engineers
That’s true. Goats are odd animals, but they’ve never been observed in the wild building rafts. They don’t even seem terribly interested in engineering. However, the skills and processes you practice whilst doing Goat on a Boat, and the areas of science you’re exploring, are similar to the work professional scientists and engineers do. For example:
This goat’s boat wasn’t so successful, and it had to take to a life raft to escape.
