Why did objects float in the water?

Does it swim or does it not swim?


A small coin goes under in the water. And a big apple? Or a piece of wood? Open a swimming laboratory and compare and observe which items are swimming.

You need:

  • larger container with water, e.g. bowl, bucket, paddling pool
  • Towel underlay
  • Objects made of different materials, e.g. stones, corks, coins, pieces of wood, plastic spoons, putty
  • Fruit / vegetables from the kitchen, e.g. potatoes, apples, raisins and grapes

This is how it works:

1 Take up everyday relevance

When bathing, washing dishes, at the lake or in the swimming pool - children see things swim or go under. In the bathtub you will experience toy figures sinking, in contrast to rubber ducks, while large, heavy ships float on the surface of the water in the harbor.

2 What floats, what doesn't float?

Take the children and collect items that the boys and girls will want to know if they are swimming. Set up a large, transparent water container and form a circle around it with the group. Now the children make assumptions about the swimming behavior of their objects and try out directly whether the objects swim or sink. Everyone should have the opportunity to look at the various objects in peace. How do swimmers differ from non-swimmers? Do this activity several times. The children will always investigate new things and thus develop a feeling for the characteristics of a swimmer.
Now add more items to the selection of objects: It is best to choose objects that behave differently than the children expect. Compare e.g. B. the swimming behavior of fruits and vegetables.
Then lay out a few things, each made of a single material, such as: B. made of wood, stones, corks, coins or metal keys. Make sure that these objects have no cavities and trap air in them. What do the children think happens to these materials in the water? Does the swimming behavior change if you z. B. choose a light and a heavy stone?

3 round or square? - Properties in the swim test

Now put together a selection of objects that have clear similarities and differences and for which there are at least five of each type. The following groups of properties are available: round / spherical objects, angular, boat-shaped, heavy, light, large, small or colored objects.
Have the children each choose an object and predict whether it will swim or not. How do you justify your decision? What properties do you think are responsible for this? Now it is tested: Are the children correct or are there any surprises? It helps the children if they are allowed to carry out this experiment several times. Repeat this suggestion and also expand the selection of objects. Also test what happens if the children deform the plasticine differently before putting it in the water.

4 things to know for adults

Whether something swims or goes under in the water depends on the density of the object and the density of the water. The density is a material property. The more an object weighs and the less space it takes up, the denser it is. If an object is denser than water, it will sink. If it is less dense than water, it can swim. The "density" is also present in our everyday language. For example, the bus driver instructs his passengers to "move closer together" so that more people can fit into the bus.
Why is a large steel ship floating now, but a small steel ball sinking? Because ships have many cavities and their weight is therefore distributed over a large space. So they have a lower density than the steel ball. This phenomenon can be made visible with modeling clay: A modeling clay ball goes under when you put it in the water. If the kneading ball is rolled out and shaped into a bowl, it floats. The weight of the putty is distributed over a larger area.

Supplementary material for your educational work

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