Support children to investigate which objects float and which objects sink.
Early Learning Goal links
Understanding the World ELG: Past and Present – Talk about the lives of the people around them and their roles in society;
Understanding the World ELG: People, Culture and Communities – Describe their immediate environment using knowledge from observation, discussion, stories, non-fiction texts and maps;
Understanding the World ELG: The Natural World – Understand some important processes and changes in the natural world around them, including the seasons and changing states of matter.
STEM vocabulary to introduce
Float, sink, sunk, sank, rise, rose, risen, size, big, small, heavy, light, bottom, top, surface, under, side, shape, predict, test
Before you start
Show the children the marine engineer poster and tell the children that they are going to be marine engineers for this activity.
Ask the children if they know what a marine engineer does. Marine engineers make and fix boats, ships, submarines, oil rigs and drilling equipment.
Tell the children about the attributes. Marine engineers are:
Curious and want to know which materials sink and float.
Creative when they design and build things like boats, ships, submarines and oil rigs.
Resilient when their ideas or designs don’t work the first time – they try again to get it right.
Tell the children that they are going to be curious like marine engineers and investigate which objects sink and which float.
You could assess how much the children already know about floating and sinking, by asking:
- What does float mean?
- What does it mean when something sinks?
- Which objects can you think of that sink to the bottom of the water tray?
- Which objects can float on the top of the water?
What to do
Ask the children to choose an object from your selection and then ask them to make a decide whether it will sink or float when they put it in the water. You could introduce the word predict and explain that a prediction is a really good guess based on what they already know.
You could ask:
- Do you think (or predict) it will float or sink?
- Why do you think that?
- Is it big or small?
- Is it heavy or light?
- What is it made of?
Tell the children they are going to test their prediction and ask the children to put the objects into the water one at a time.
After testing, you could ask:
Did your object float or sink?
Was your guess or prediction right?
Continue to test all of the objects in your selection. Ask the children to sort them into objects that float and objects that don’t float.
You could ask:
Do big objects always sink?
Do small objects always float?
What is the same about the objects that floated?
What is the same about the objects that sank?
Do heavy things always sink?
Do some materials (e.g. plastic, wood, metal…) always float?
Do some materials always sink?
If you were a marine engineer, which material do you think you would choose to build a boat and why?
Other things to try
Challenge the children to find natural items in the outside area will float.
You will need:
- A water tray, bucket or bowl
- Access to sticks, leaves, flowers, grass, pine cones, acorns etc
To extend play, you could ask:
- Do you predict that (object) will sink or float? Why?
- Why do you think that is floating/sinking?
- Do you think it would make a good raft or boat? Why?
- What could you do to test whether it would make a good boat?
The science of floating and sinking
We have put together some useful information about the science of floating and sinking to accompany this activity. Don’t worry this is for your information only and to help you answer any questions children may have. We don’t expect you to explain this to the children in your setting!
Why do objects sink or float?
There are two forces acting on objects in the water. The weight of the object pulls down, while the upthrust of the water pushes up. If the weight of the object is equal to, or less than, the upthrust, it floats. Things that float are called buoyant. If the weight of the object is greater than the upthrust, the object will sink.
What is upthrust?
This is the upward force pushing towards the surface. When you try to hold a beach ball, empty plastic bottle or inflatable under the water, you can feel this force pushing it towards the surface. The upthrust force is equal to the weight of water displaced by the object.
Why do some heavy objects float?
An object floating on the water takes up space. To make this space, some of the water is displaced – pushed out of the way. The amount of water displaced is the same as the volume of the object that is below the surface of the water. The more weight you add to the object, the more water it displaces. As the volume of water displaced increases, the upthrust force increases to balance this, and the object floats. There will eventually be a point where the weight of the object becomes greater than the upthrust and the object sinks. To see displacement, you can fill a container almost to the top, then add stones to it, you can see the water level rise as the stones take up space in the water.
The shape of an object will determine if it will sink or float. A lump of iron, such as an anchor, will sink as it is dense and all of its weight is concentrated in one place. This makes its weight greater than the upthrust pushing against it, and so an anchor will sink. If the same amount of iron is made into the hull of a boat, it is spread over a greater area.
The larger boat displaces more water than the anchor and so the upthrust is greater. This means that the same amount of material will float in the shape of a boat, but will sink if it’s in a more compact shape.