How can big, heavy things like boats made of steel float? How come some light, small objects like paperclips sink? (Actually, you can make paper clips float – will show you how one day!). This exploration combines two of my favourite things: clay and marbles. It helps explain how boats can be made out of heavy materials such as steel but can still float and links beautifully with our tinfoil boats investigation.
What do you need?
Clay, plasticine or playdough
Tank of water
What’s the STEM
Whether an object floats or sinks depends on how much water it displaces (or pushes out of the way). Objects that displace a lot of water are buoyant. They receive an upward push from the water. This upward push supports the object so that it floats. If an object displaces an amount of water heavier than the object itself, the object floats. If the water displaced weighs less than the object, the object sinks. A large ship (which has a lot of space inside it) pushes a lot of water out of the way so it is very buoyant.
Where to start?
You can test lots of different items to find out if they sink or float. You might even like to weigh some things first. Talk about why things sink or float. What do the children think?
Then introduce a big ball of plasticine and some marbles. Does the ball sink or float? How much does it weigh? We can flatten it out a bit to make it float. Does it still weigh the same?
What can you do with the marbles? Do they float or sink? What happens if you put them on the floating plasticine?
The Inquiry Cycle* is a great tool to guide you through your research with the children. This will help you get into the habit of discussing and forming hypotheses, testing and playing with the ideas, documenting and DICUSSING the results. And then, of course, asking, ”Why?” Investigations like this are a great opportunity for you to support the children in developing those important skills – thinking, analysing, discussing, hypothesising and observing.
Experimenting with weighing items and changing their shapes might give the children an idea that it is not exactly the weight that is the deciding feature in whether something will float or not. However, density is a tricky concept. We have a few floating and sinking activities investigating fruit and liquids in our Little Scientists at Home series that will help develop the concept of density in children’s minds.
There are many children’s books that joyfully explore the laws of physics. Reading books such as Who sank the boat by Pamela Allen might start some great investigations or give the children new ideas for their hypotheses and discussions.
If you’re looking for more inspiration for inquiry-based projects, our book Everyday STEM inquiry in practice, includes a wonderful example of sinking and floating explorations in an early childhood service, as well as many other STEM projects.