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Why do heavy boats float?

Posted on January 14th, 2021 in STEM gems
Large freighter (boat) on ocean


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?

What next?

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.

Inquiry cycle

The Inquiry Cycle is a tool used by Little Scientists to guide the research process, which is divided into different phases of thought and action that typically occur in a repetitive cycle. If you would like to learn more about Little Scientists’ approach to research in early childhood, you might be interested in attending one of our workshops. The Water workshop is a great introduction to our approach and ties in well with this article.

About the author:
HAYLEY BATES, National Certifications Coordinator

Hayley Bates

Hayley has an insatiable thirst for learning – about everything! Her sheer joy of discovery and passion for professional development makes her the perfect person to run the Little Scientist’s House Certification program.

Never happier than seeing what happens to balloons in the freezer or exploring the projects submitted by services for certification, her enthusiasm is complemented by her background in science and maths making her the ideal coordinator for our Little Scientists Houses.

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