Buoyancy

Buoyancy is a fancy word we use to talk about whether something sinks or floats. Why do you think some things float and some do not?

Required Materials

Tin Foil

Introduction

Visual	Discussion
	Look at this picture of a boat filled with people (show picture). It floats. What about another way of traveling: a car? Would a car sink of float? We are not going to try it, but here is a picture of someone who did (show car picture).
	Let’s think about the picture of the boat and the car. They are both pretty big and heavy. They both will carry people. They both are made of lots of lots of things, including metal. Why does one sink and the other float? Now let’s do some testing of our own.

Will It Float?

Set out a bowl of water and various objects. Hold up the first object. Have the students predict if it that object will sink or float. Ask a student to place the object in the water. How good were they at predicting what would float? Why do they think some things sink and others float?

Visual	Discussion
	Push a book across the table. Why did the book move? Have a student push the book back to you. Why did the book move back the other way? The book moved because I pushed on it and nothing really pushed back. When a force pushed back, the book went the other way. “Force is pressure or energy exerted on something that will make it move.”
	Have a student drop a book on the floor. Why did that object fall? Have them drop it again, but put your hand under the book and hold it up. Why didn’t it hit the ground this time? Show picture of astronaut. What is in this picture? What is the astronaut doing? Floating! If the book were in space, it would float too. On earth though, the book drops because gravity exerts a force on it, pulling it toward the ground. When I put my hand under it, my hand exerted pressure back up against the book, holding it up. “Gravity is the force by which a planet or other body draws objects toward its center.”
	Have two children arm wrestle. How was one person able to push the other person’s arm down? Next, try using equal force so the arms don’t move. If both players use equal force, the arms don’t move. If a person exerts more force, they can push the other person’s arm over. In science, there is an important rule. “The strongest force wins!”
	Let’s think about the picture of the car again (show picture). The car sunk because the force of gravity on the heavy car was strong enough to pull it down. So why didn’t gravity pull the boat under? There has to be a force pushing up on the boat that is greater than the force pushing up on the car. That force is water. Gravity pulls down, but water pushes up. This is called buoyancy. “Buoyancy is the upward force exerted by a liquid or gas on an object.”

Understanding Buoyancy

Tin Foil Boat

Cut tin foil into 2 squares that are the same size. Wrap the tinfoil in a ball around the penny. Does it sink? Lay the penny on a flat piece of tin foil. Does it sink? Use the tinfoil to make a boat. Does it sink? See how many pennies you can add before it sinks.

Visual	Discussion
	Here’s another rule (show word strip): The amount of water pushed aside or “displaced” by an object equals the amount of force that the water pushes upward on the object. That means that objects that are heavy for their size sink; objects that are light for their size float. We say that objects that are heavy for their size are “dense”.
	The ball of foil and the penny have the same weight as the flat foil and penny. However, the ball of foil takes up a smaller amount of space—it is heavy for its size. Gravity pulls the dense foil ball down (show word strip).
	The flat foil takes up a lot of space (or surface area) on the water. It pushes aside (or displaces) a lot of water—it is light for its size. The water pushes up on the foil with an equal amount of force—enough to keep the foil (even with pennies on it) from sinking (show word strip). Similarly, a big boat floats while a car sinks. Both are heavy objects, but the boat is designed in such a way that it pushes or displaces a lot of water—enough that the force of the water pushing up on the boat keeps it from sinking.

Predictions

Orange Predictions

Predict what will happen if you put an orange in water. What will happen if peel the orange and put it back in the water? Try it out.

The orange peel contains lots of air pockets that made the orange lighter for its size, so with the peel the orange floated. When the peel was removed, the orange was heavier for its size and sunk.

Think About It

Which do you think floats better, a heavy person or a little baby? A person laying on their back, or upright?

A large, fat person weighs a lot. A little baby doesn’t weigh as much. However, it is likely the baby weighs more for their size than the fat person. A baby has a lot of bone and muscle. A fat person has lots of fat cells. Fat weighs a lot less than bone and muscle, so a fat person may float while the baby sinks like a rock. A person laying flat on their back in the water will float better than when they are upright. Their weight stays the same but the amount of water they are displacing when they are laying on their back is much greater. This is also why life jackets work. Life jackets are small, but very buoyant. Because people are made up of a lot of water and air, it does not take very much extra buoyancy from a life jacket to make them float. Next time you are in a pool, experiment with these concepts.

Experiment & Try It Out

Pick an activity and try it out as a group or at home

Clay boats

Give each person three balls of modeling clay that are the same size. Have them form the clay into three different shapes—a ball, a log and a boat shape. Which floats? Which shape displaces the most water or takes up the most space in the water? Which is “lighter for its size”?

Cardboard Boat

Divide into teams. Give each team some cardboard, cord and duct tape. Give them 20 minutes to design a boat. Test the boats in a kiddie pool, bathtub or nearby pond. Which floated? Try putting cans in the boat. How many cans can the boats hold before they sink?

Nature Journaling

Go someplace that has a river, lake or other body of water. Try throwing different items in the water and see what floats and what sinks. Draw what occurs. Record answers to the following in your nature journal: