Featured Project: What Makes Objects Float?

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

  • 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

 

Concept 1 – Water Displacement

Materials:

  • 1 clear bowl or container
  • Water, enough to fill the container
  • Object that will fit inside the container (some examples are toys, aluminum foil ball, plastic easter egg, bar of soap)
  • Towel, to clean up any spilled water

Instructions:

  1. Fill the container with water nearly to the rim.
  2.  Place the object in the water.
  3.  As the object enters the water observe the effect it has on the water level. 
  4.  Remove the object from the water, and as the object leaves the water observe the effect it has on the water level.

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

Materials:

  • 1 bowl or container, deep enough that an egg can sink completely
  • 1 egg, uncooked
  • Water
  • Table salt
  • Towel, to clean up any spilled water

Instructions:

  1. Fill the container with water. Add enough water that the egg can sink and be completely covered.
  2.  Gently place the egg in the water. Observe if it floats or sinks.
  3.  Remove the egg.
  4.  Add salt to the water and stir well. The amount of salt you add will depend on how much water is in your container… the liquid should be very cloudy and very salty.
  5.  Once the salt is mixed in, gently place the egg in the saltwater. Observe if it floats or sinks.

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

Materials:

  • Paper
  • Pencil
  • Aluminum Foil
  • Pennies
  • 1 large wash basin or tub
  • Water
  • Towel, to clean up any spilled water

Instructions:

  1.  Make 5 equal sized sheets of aluminum foil (about 12″ long).
  2.  Use each sheet to build a boat – and make each boat a different design. Experiment with different surface areas on the bottom of the boat, different shapes, different sizes. In the end you should have 5 unique aluminum foil boats of various shapes/sizes, but each made from the same amount of aluminum foil. Each boat will weigh the same because they are made of equal amounts of aluminum foil.
  3.  On your paper, draw a table with 6 columns and 2 rows. Fill it in like the picture below.
  4. One at a time, place a boat in the tub of water. Add pennies to the boat until the boat sinks. 
  5. Record the number of pennies it takes to sink the boat on your record table. Repeat for the next boat. Be sure to keep track of the order of the boats!
  6. Determine which boat held the most pennies before sinking. What do you notice about the properties of that boat? Was the bottom bigger/flatter? Did it have a larger volume (did it take up the most space)?
  7. Make 1 last sheet of aluminum foil (about 12″ long).
  8. Crumble the last sheet into a ball and place it into the tub. Does it sink or float? 
buoyancy_table
What do your observations tell you about the relationship between weight, volume, surface area, and buoyancy? Two objects may weigh the same, but if one object has a larger volume it will have more buoyancy. 
 
How does this relate to buoyancy? Buoyancy (amount of force that the water pushes back) will be larger when the volume is larger. 
Great job working through these concepts! To come back to our original equation for buoyancy:
 
Buoyancy = Density * Volume * Gravity
  • 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.

References