Welcome back! For our latest demonstration highlight, we’re exploring the concept of buoyancy, and a few of our demonstrations that let us see buoyancy in action in the classroom.

Technically speaking, buoyancy is the upward force that a fluid exerts on an object that is immersed (partly or wholly) in it. This is the force that determines whether things float or sink, and why some things feel lighter when you hold them in the water rather than in air.

First developed more that 2200 years ago, Archimedes’ Principle (in more modern terms) states that the upward force (or buoyant force) on an object submerged in a fluid is equal to the weight of fluid that it displaces. This means that if the average density (the mass per unit volume) of an object is greater than that of the fluid, it will sing, as the force of gravity on it is still greater than the buoyant force. If its average density is less than that of water, it will float, as the buoyant force is greater than the force of gravity.

 two cylinders hang from a spring scale over a bucket of water

We can show the physics behind this in the classroom with demonstration F2-01. Hanging from a spring scale are two cylinders of equal volume – one solid metal cylinder, and one an empty bucket. We lower the solid cylinder into the water, and can measure the buoyant force as the change in weight on the scale. Then, we pour water into the empty cylinder. A volume of water has a weight equal to the buoyant force on an object of that volume, so when we’ve poured in water to equal the volume of the solid cylinder, the weight shown on the scale is back to where it was when both cylinders were hanging in the air!

Conversely, consider demonstration F2-05. A lightweight model boat containing a small, dense weight is floating in a tank water. We can use tape to make the height of the water on the side of the tank. Then the weight is removed from the boat and placed in the bottom of the tank, and we see that the water level on the side of the tank goes down, as the boat floats higher in the water.

a clear plastic boat floats in a clear glass tank of water. A lead weight rests in the boat.

The weight is denser than the water. When it is sitting in the bottom of the tank, it displaces an amount of water equal to its fairly small volume. When it is in the boat, though, in order to float the boat-and-weight combination has to displace enough water to match the mass of this boat-rock system, pushing the level in the tank higher. When the weight is removed, the average density of the boat is reduced.

You can experiment with this in the classroom or at home with this Buoyancy Simulation by physics teacher Tom Walsh. Try changing the density of the submerged object and the density of the fluid, and see how it floats! You can have the simulation add a free-body diagram to show you the forces in action.