Buoyancy Explained: Making objects float on water

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Buoyancy can be a tricky concept to explain to a young kid. It’s easy to think that the weight of an object determines its ability to stay afloat or sink in the water, but it’s much more than that!

This STEM experiment demonstrates the buoyancy of a small rock by placing it on different sizes of foam paper. It helps to understand the concept of displacement and buoyancy in a very hands-on way.

How to make the Buoyancy Explained experiment

Supplies you will need

For this experiment, you’ll need:

Foam paper
A small rock
Tub of water
Scissors
Optional: a lighter object, like an acorn

Supplies needed for the Buoyancy Explained STEM experiment

Before you start

It could get messy with the tub of water, so be sure to put a towel down if you want to contain the mess.

Also, if you are using a glass bowl like me, be gentle placing the rock in the water. We don’t want your glass bowl to crack!

Instructions

Here is how to do this experiment with your child:

Step 1: Cut out varying sizes of foam paper

Part of the experiment is seeing how much water we need to displace for heavier and lighter objects.

When we use a larger piece of foam paper, we displace more water to support more weight. When we use a smaller piece of foam paper, we displace less water to support less weight.

You can choose to cut out varying sizes of foam paper now, or cut as you go with the next couple of steps.

Step 2: Test out different sizes of foam paper with a heavy object (rock)

Have your child decide what size foam paper they would like to use. If you are using a rock and a lighter object (like an acorn), ask if they think we need a larger piece of foam for the rock or the acorn.

Test your hypothesis!

A rock floating on a piece of foam paper

Step 3: Test out lighter objects (optional)

We had an acorn handy, so we decided to test that out too. Once my daughter experimented with the rock, she knew she wouldn’t need a large piece of foam to support the weight of the acorn.

We started with a medium-sized piece of foam and continued to cut it down to see how small we could get the foam paper and still support the acorn.

An acorn floating on a piece of foam paper

The STEM behind the Buoyancy Explained experiment

This experiment teaches:

Buoyancy
Displacement
Observation

How it works

This STEM experiment demonstrates buoyancy and displacement by testing out objects at different weights to see how much water to displace to make them float. The larger the piece of foam paper, the more water is displaced.

The weight of the displaced fluid (water in this case) is equivalent to the magnitude of the buoyant force.

Since the rock is heavy compared to an acorn, the rock will need a larger piece of foam to support its weight and make it float.

Buoyancy

Buoyancy is like a superpower that helps things float in water. Some things have enough of this superpower to stay up on top of the water, and some things don’t, so they sink.

We need the water’s upward force, or buoyancy, to keep us afloat. It’s through buoyancy and displacement (in the next section below) that large ships can easily float on the water.

How did the concept of buoyancy come to be? It involves a crown and a bathroom floor soaked with water…

King Heiron II called on a Greek physicist and mathematician named Archimedes to find out if his crown was really made of pure gold, or if he was tricked and the crown was made of gold and silver.

Archimedes filled a bathtub up completely and experimented with how much water was displaced (essentially how much water poured out of the tub) when he placed a piece of silver of equal weight to the crown in and then a piece of gold equal to the weight of the crown in the tub.

The silver displaced more water than the gold, so it was time to test the crown.

When he placed the crown in the water, it displaced more water than the piece of gold, showing that the person who made the crown mixed silver and gold to trick King Heiron II!

Displacement

Displacement is the volume of fluid that an object pushes aside when it’s submerged in water.

Think about little water molecules, hanging out in a pool. When we drop a pair of goggles into the water and they sink, those water molecules have to move and make way for the goggles. Or, in the case of an object floating, the water molecules at the water’s surface have to go somewhere when they’re pushed aside by the floating object. That’s displacement!

To explain displacement, you can first ask a very engaging question: Why does a large ship float on the water and a tiny pebble sink?

To answer that, we have to keep in mind that the weight of the displaced fluid (water in this case) is equivalent to the magnitude of the buoyant force.

Even though the ship is very heavy, it is shaped in a way that spreads its weight out and pushes a lot of water away, which helps it float by producing a larger force of buoyancy on the ship. A little pebble doesn’t push away much water because it’s small, so it sinks.

Observation

Kids will have to closely observe the rock to see if the foam paper will support its weight.

You can also take it a step further and figure out exactly how much foam paper your rock needs! Cut off a small piece of the foam paper and test to see if it holds the rock. If it holds the rock, cut off a little more. Keep cutting off small pieces to see how much foam paper you need for your rock.