Why does a tiny pebble sink to the bottom of a lake but a gigantic cruise ship floats on the high seas of the ocean? Sometimes it can seem like the things that float and sink really don’t make sense. Buoyancy describes the tendency of an object to float or sink in a fluid. In this featured project, we will explore the world of buoyancy and do 3 experiments to help demonstrate some of the key concepts.
Have you ever played the teamwork game where you sit back to back with a buddy on the ground, link arms, and have to work together to stand up? When your partner pushes against your back, you have to push back equally as hard to keep upright!
This concept of equal and opposite force is very similar to buoyancy.
Think of buoyancy as a teamwork game between the water and the object. The object, because it has weight, is pushing down into the water. The water, because it’s being pushed, has to push back! The force of the water pushing back is called buoyancy, and it’s that force that can keep things floating. There are a few concepts that will help you understand what factors are at play. In this featured project we will discuss displacement, density, and weight/volume/surface area.
The equation to calculate buoyancy is:
Buoyancy = Density * Volume * Gravity
Concept 1 – Water Displacement
What did you notice? When the object enters the water, does the water level rise? When the object leaves the water, does the water level go back down?
This concept is called displacement. The water and the object you are holding cannot both be in the same space at the same time. When you put your object into the container, it pushes the water out of the way – or displaces it. A bigger object will displace a bigger amount of water, and a smaller object will displace a smaller amount of water. Try out some other objects to test this theory!
How does this relate to buoyancy? Buoyancy (amount of force that the water pushes back) will be larger when the volume of the object is larger.
Concept 2 – Density
Did the egg float in the salt water? If it didn’t, add more salt until it does. What does adding salt do to the density of the water? It increases the density of the water. Once the water density is much higher, the egg will float.
How does this relate to buoyancy? Buoyancy (amount of force that the water pushes back) will be larger when the density of the fluid is larger.
Concept 3 – Weight, Volume and Surface Area
- Density of the liquid (usually just the density of regular water, unless it’s the ocean and it would be saltwater)
- Volume of the object that is underwater
- Gravity is used because gravity is what causes an object to have weight
So the buoyancy (amount of force that the water pushes back) is based on the density of the water it’s in (buoyancy will be larger if the water is more dense), the volume of the object that is underwater (buoyancy will be larger if the object has larger volume) and gravity.
Now that you’ve learned these concepts, think of your own ways you can play with buoyancy in the world around you! Perhaps the next time you swim in a pool, take a bath, or do the dishes you can test the forces of buoyancy with the objects around you.