Skip to main content

Buoyancy and Floating Lesson Plan

  • Author:
  • Updated date:

I am a Christian. I was an 8th-grade American History teacher. I am currently a freelance writer, public speaker, & homeschooling mom of 9.

Testing out aluminum foil barges to see which design can hold the most cargo (pennies)

Testing out aluminum foil barges to see which design can hold the most cargo (pennies)

This is part 1 of a 5 part hands-on unit study on Floating & Flying (Fluid Mechanics). This week's focus is buoyancy (floating). Build an aluminium foil barge that can hold the most pennies, experiment with what floats and sinks and why, explore the relationship between density and buoyancy, and more! My lessons are geared toward 4th-5th grade level children and their siblings. These are lessons I created to do with a weekly homeschool co-op. We meet each week for 2 1/2 hours and have 33 children between the ages of 1-13. Use these fun lessons with your classroom, family, after school program, camp, or co-op!

Please DO NOT copy this elsewhere without giving proper credit: .

(Some photos on this page were taken by one of the mothers in our co-op

who operates Michelle Harrison Photography.)

Just before the training wheels came off

Just before the training wheels came off

Devotional: Jesus Walked on Water

1. Pray. Read & discuss Matthew 14:22-33. Say something such as:

-Have you ever had to try something hard...maybe something you were a little afraid to do at first? For my fifth birthday my father took the training wheels off my nice, pink bicycle. I was excited about riding, but I was so scared of falling off and skinning my knees. Thankfully my father was there to help. Have any of your parents helped you learn to ride a bike? As I was learning he'd walk alongside as I pedaled, holding on so that I wouldn't fall - and then letting me go just a little bit at a time. When he thought I was ready, he let go. The first few times he let go, I got so nervous that I did start to fall, but he was always there to catch me. Eventually with a few days of practice in the grass, I was able to ride up and down my cul-de-sac with ease!

-Some things sink and some things float. We can look at something and know from its weight, size, and so forth whether or not it will float or sink. What if I put this rock in this water? Would it float or sink? (Drop it in.) What about if it didn't sink? What would we call that? (A miracle.) When something happens that is contrary to all the rules of science, we call it a...

-The Bible passage we just read tells us about one of Jesus' most famous miracles. His disciples were on a boat, and they were got scared because the waves were getting higher. Suddenly they spotted saw Jesus. He was walking toward them on top of the water! At first they thought he must be a ghost. Jesus knew they were afraid and told them not to worry. Peter wanted to know for sure, so he asked Jesus to help him to walk on the water too. Would you be afraid to try?

-Peter stepped out of the boat. What happened? He began walking toward Jesus – right on top of the water! But just like my first few hours on my 2-wheeled bike, when he saw that he was really doing it, he got nervous and started to sink! Thankfully just like my dad was there for me on my bicycle, Jesus was there to catch Peter.

-Is it possible for you or for me to walk on water? Today’s devotional lets us know that Jesus really was God. He could do things that are impossible for people to do, such as walking on the water. This true story also helps us to understand that when God asks us to do difficult things, He will be there watching and supporting us. Isn’t that encouraging to know?

YOU WILL NEED: a rock and a container of water

Determining what floats and sinks and why

Determining what floats and sinks and why

What Floats? What Sinks?

***If you have 16 or more children, divide the children into 2 groups. Have half the children do activities 2a-c while the other group does activities 3a-e. Then have them switch. ***

2a. (Prep: Fill 4 containers of water.) Ask, “What makes things float?” Let a few children share their ideas. Divide children into 4 groups of 3-4.

-Give each group a bowl/container of water and some objects. Give each child 1 of each item.

-Ask the children to make a hypothesis by dividing the objects into what 2 groups: items that will float in water and items that will sink in water. After the groups have divided their items, have them each use some words to describe each of their piles. (For example, they might say “heavy” items will sink while “light” ones will float.)

Scroll to Continue

-After the children have shared their descriptions, hold up a penny and an apple. Ask, “Which item will float?” Demonstrate that the apple floats in water and the penny sinks.

-Ask, “Which one weighs more?” If the apple weighs more than the penny, then why does it float? We’ll explore that concept today.

-Allow children to now test out their 2 piles and see if they were correct about which items float and which sink.

YOU WILL NEED: 1 apple, 1 penny, & 12 sets of various items for children to test out in the water [such as 12 paper clips, 12 toothpicks, 12 pennies, 12 bottle/milk caps, 12 marbles, 12 plastic beads, 12 sponge pieces, 12 pencils, 12 pieces of aluminum foil, 24 pieces of paper, 12 small balls, 12 pieces of Styrofoam, 12 plastic spoons, 12 wooden spoons, 12 twigs, etc.] & clementines (brought by person doing activity below), 4 containers for water (like plastic shoe boxes) & towels


Altering and Defining Buoyancy

2b. After the children have tested the buoyancy of all their items, find out if all the groups came up with consistent results. (“Did the marble sink for everyone? How about the plastic beads? The aluminum foil?”)

-If there are differences in their findings, ask the children to speculate why this could be. How could they explore this further? Did the difference have to do with different procedures?

-Ask, “Is there a way that you could change some of the sinkers into floaters or make the floaters into sinkers?” If no one has ideas, ask, “What would happen if you changed the shape of the aluminum foil? What happens if you peel the orange?” Try it!

-First have the children crumple the aluminum foil into a tight ball and see if it floats or sinks.

-Then have one of the children peel their orange and drop it in the water the same time as another person drops in their unpeeled orange.

-Explain: An unpeeled orange floats because its skin has pockets of air. Ducks and geese float for a similar reason. Their feathers contain tiny tubes filled with air. Sometimes young swimmers use air-filled water wings to stay afloat. When you removed this layer of air pockets (the peel) from the orange, it sinks a bit, just like what would happen if you removed water wings from some young swimmers. (Note: According to some books, the unpeeled orange was supposed to sink. It didn’t. However, it didn’t float as high as the unpeeled orange.)

-Make sure all items have been removed from the water and placed on towels to dry.

-Sanitize children’s hands. Allow all children to peel their oranges and eat them. Assist children who need assistance in peeling their oranges.

YOU WILL NEED: 24 clementines (preferred) or other easy to peel orange/tangerine, 24 napkins, and hand sanitizer

2c. Refer back to the words the children originally used to describe items that float or sink. Ask the children to look for commonalities among the items that float and those that sink. Which descriptive words would they change? Are there words they would add? Introduce vocabulary: objects that float can be described by a new word, “buoyant.” Have everyone say the word, “buoyant.” Ask, “What does it mean for an object to be buoyant?”

2d. Only if the other group isn’t done yet,begin to read a book on buoyancy,Things That Float and Things That Don't by David A. Adler.



3a. Let children pass around 4 identical boxes that have been filled with different materials. Let children handle the boxes, but do not let them open the boxes. It would be a good idea to tape them shut, anyway, to prevent leaks.

-After everyone has held all the boxes, ask them to compare the features of the four boxes. They should note that the boxes look identical. Tell them that another way of saying that the boxes are all the same size is to say their volumes are identical. Have the children say, “volume.”

-They should also note that the boxes had different weights. Tell them that another way of saying that is to say they have different masses. Have the children say, “mass.”

-Once these two observations have been made, you can let the children know there is one word that includes both of those observations. That word is density. Have them say the word, “density.”

-Density tells us how much material is packed into a given amount of space. It tells us how the masses of two different objects would compare if they both had the exact same volume. The dry cereal (or popped popcorn) is not very dense. The material in the box does not weigh very much, because there is a lot of air occupying the spaces between the cereal flakes (or between the popped corn kernels, which themselves contain a lot of air, too). The sand and rice grains are denser; they can pack together more tightly in the box. Sand grains can pack very tightly together, and so the box containing them is the densest of all.

YOU WILL NEED: 4 identical containers (like macaroni & cheese boxes or Jiffy cornbread mix boxes) each filled with a different substance that would make them weigh different amounts (such as 1 filled with popcorn, 1 filled with rice, 1 filled with sand, and 1 filled with cereal) and securely taped closed

This cartoon provides a good explanation of volume and density.


Density: Mass Per Volume

3b. Ask the children how it could density can be quantified.

-Ask: “What measurement do we use for describing length?” (meters, feet, inches, etc.). “What do we use for measuring temperature?” (degrees Fahrenheit or Celsius) “What do we use for describing speed?” (miles per hour)

-Ask: “What units do you think we should we use for describing density if it is the amount of material that fits into a given volume?” Remind them of the 2 words you had them say before: mass (or weight) and volume are the two quantities needed. Density is defined as the amount of mass per volume of a substance. Per always means divide. They are already familiar with speed: to find the speed, they divide the miles traveled by the hours it took to travel them. This division results in the speed expressed as miles per hour.

-Divide the children into 4 groups. Give each group a box. Have the children use rulers to measure the sides of the boxes. On a sheet of paper [you, the parent] write the 3 measurements and multiply them together to get the volume of the box.

-Have each group weigh their box on the kitchen scale to determine their weight/mass. You, the parent, will write those on the paper.

-Now try to lead the children to help you write out the density of each box (including the units, oz./in3). Scientists really use grams and centimeters, so the measurement for density is really g/cm3. The heavier box should, of course, have a higher density.

YOU WILL NEED: 1-4 kitchen or postal scales (the kind that can weigh 1-16 ounces), 2 sheets of paper, a writing utensil, & 4 rulers (brought by families)


Density & Buoyancy

3c. (Prep: Fill 4 containers of water. Pick the largest ones available, preferably larger than shoe box size) Ask, “How do you think density relates to buoyancy, or the ability of an item to float?” Let children share their ideas.

-Pass golf balls and ping-pong balls. They have similar volumes but one is much heavier, therefore, more dense. Ask, “Which item do you think will float? Sink?”

-Give each group a golf ball, ping pong ball, and container of water. In each group let child #1 drop the golf ball and child #2 drop the ping pong ball in a bowl of water and see if they were right.

-Have children lay the balls on towels to dry.

YOU WILL NEED: 4 golf balls, 4 ping pong balls, 4 containers of water (like plastic shoe boxes) & towels


Density, Buoyancy, & Coke Ingredients

3d. It’s not always obvious which item is more dense. Pass out a can of Coke and a can of Diet Coke to each group. Ask the children if they both have the same volume. (Yes.) Do they have the same mass? The same density? Let’s find out.

-Ask them to form a hypothesis. Tell them to say, “hypothesis.” Ask, “What is a “hypothesis?” (an educated guess). Ask, “Will both sink, both float, or one sink and which one?” Let all the children vote to show their hypothesis.

-In each group have child #3 put in the coke can and child #4 put in the diet coke can. If you do not have containers that are significantly deeper than a shoe box size, use your pitcher of water and drop the Coke & Diet Coke cans in so that the children can see the dramatic difference in one floating higher than the other.

-What happened? Based on what you just saw, which can of soda has more mass? (non-diet) How did you know that? (It sank lower in the water.) The denser can must have more because you know they have the same volume. The regular Coke has more “stuff” in the same sized can, though. Can anyone guess what the difference might be. (It is the corn syrup in the regular Coke that makes it more dense than the diet Coke which contains artificial sweetener.) (*Note: You need to use the 4 largest containers of water for this demonstration as the shoe-box size containers of water weren’t able to demonstrate this as much. The Diet Coke cans we used did not sink, but they did float lower than the cans of regular Coke when we used a large container of water like a medium size plastic storage bin.)

-Have children lay the cans on towels to dry.

YOU WILL NEED: 4 cans of Coke, 4 cans of Diet Coke, 1 large, clear pitcher filled with water – the deeper it goes, the better, 4 containers of water (like plastic shoe boxes) & towels

Watch Bill Nye do the Coke demo. I think Coke has changed their ingredients because we could not get ours to sink.

More Good Books on Buoyancy

Also look for the cute story What Floats in a Moat? Lynne Berry.


Buoyancy & Surface Area

***If the children were divided into 2 groups, have them come back together into 1 group. ***

4. Ask the children, “How does a boat or ship float carrying hundreds of pounds worth of cargo while that same cargo would sink to the bottom of the ocean if dumped overboard?” Let them share their ideas.

-Ask them who went swimming this summer. Ask if anyone is able to float on their backs in the water. Allow for 1 or 2 children to describe what they do to float in the pool.

-When you are in a pool and you stretch out your body, you will float. But if you wrap your arms around your legs and curl up into a ball, you will sink. It all has to do with how much water is pushing against you and a little scientific principle called buoyancy. Ask, “Have you ever heard that word buoyancy? What does it mean?”

-When you stretch out flat, more water pushes against you since your body is laid out flatter and has more surface area. Have the children say, “surface area.”

-Have the children stand up, stretch out their arms and spread out their legs and say, “More surface area.”

-When you curl up into a ball, your body is more compact and less water is pushing against you. You have less surface area. Have the children squat down low and curl up into a ball and say, “Less surface area.”

-Ask the children to show you more surface area again. Have them show you how to have less surface area.

More children's books on why ships float

If you have young children, you can also look for the picture books Why Does It Float (Just Ask Book) by Chris Arvetis, Float and Sink (Simple Science) by Maria Gordon, and How Does A Ship Float? (How? What? Why?) by Jim Pipe.


Surface Area & Boats

5. (Prep: Fill 8 containers of water.) Group children into groups of 3-4, and have each group share a container of water. The children will also need their modeling clay.

-Have the children lay the cylinder-shaped piece of clay into the water. (If it isn’t already cylinder shaped, have them first roll it into a cylinder shape.) What happens?

-Now have them roll the clay into a ball and place it in the water. What happens?

-Have them take the same piece of clay and fashion it into a flat-bottomed boat shape and then place it in the water. What happens?

-Ask, “Why did the first 2 shapes sink and the boat (ship) shape float?” Let children share their ideas.

-Explain that if the total area of the object that makes contact with the water is large enough, the object floats. The object must make room for its own volume by pushing aside, or displacing, an equivalent (or equal) volume of liquid. The object is exerting a downward force on the water and the water is therefore exerting an upward force on the object. Of course the floating object’s weight comes into play also. The solid body floats when it has displaced just enough water to equal its own original weight. This principle is called buoyancy. Have the children again say, “buoyancy.”

-The lump of clay pushed down on a small area of water, so it sunk. Why do items that are hollowed out seem to float best? The ship-shaped clay pushes down on a bigger area of water. The water can push up against more of the clay and so that the ship-shaped clay can float.

YOU WILL NEED: clay (do NOT use play-doh or self-hardening clay), containers of water (like plastic shoe boxes), & towels


Boats and Cargo

6. Let’s consider the cargo in the ships or barges. Have everyone get their pennies.

-Ask, “What is a hypothesis?” (An educated guess) Tell them to make a hypothesis: Will the pennies sink or float? Have them drop a few pennies in the water.

-Ask, “Is there a way you could use your clay boat to help the pennies float?” Let the children try.

-Ask, “What happens if you put all the pennies on only one side of your boat?” Try it. It will start to sink! A ship’s captain has to be careful to place his/her cargo evenly on the ship. They usually start in the middle and work their way outward.

-What happens when you add all the pennies into one clay ship? Try it. Tell them, “You’re increasing the mass without increasing the volume. Your ship is going to sink!”

YOU WILL NEED: pennies


Barge Building Contest

7. Ask children what they learned about buoyancy, density, and surface area/displacement. (Let them answer.) Tell them that we are going to apply what they learned to barge-building.

We will have a contest to see who can build a boat hull that will hold the most mass while staying afloat. Children will want to achieve the highest loading capacity. Have the children repeat, “loading capacity.” Ask them what they think “loading capacity” means.

-Give each child a 12″ x 8″ piece of aluminum foil. They can test their ship as many times as they would like before the final “contest” but they may not get a new piece of foil. If their boat tears, they will have to repair it. Parents should help the youngest children.

-When deciding on the winners, divide the children into 3 groups by approximate age (oldest, middle, & youngest) for their competition group. (We had 6 and under, ages 7-9, and ages 10+.) From each age group select the barge that can hold the most pennies.

YOU WILL NEED: 24 sheets of aluminum foil that are all about the same size (about 12″ x 8″), pennies, water containers (some that are larger than plastic shoebox size), & towels


8. Have the 3 “winners” show off their barge designs.

-Ask, “What can you say about your hull design? What is the equivalent ‘boat’ to your design? (Canoe, barge, speedboat, etc.) How did the placement of pennies affect the number the boat holds?”

-You can let them know that best designs are flat barges with small sides and larger surface areas. The placement of pennies also makes a difference in the number held.

-Tell them that the record for one sheet of 8″ x 12″ foil is around 280 pennies.


Noah's Ark

9. Ask, “Who can name a boat design that is mentioned in the Bible?”

-Read Genesis 6:15-16.

-Ask the children, “When you were designing your aluminum foil ships, were you trying to design a fast-sailing ship? What was your main goal?” No, your goal was to have them hold the most cargo without sinking. That was God’s goal when giving Noah the design for the ark.

-The dimensions were at least 135 meters long (300 cubits), 22.5 meters wide (50 cubits), and 13.5 meters high (30 cubits). That’s 450 feet long, 75 feet wide, and 45 feet high!

-We’ll draw a small scale model of the ark. Have each child get their piece of paper, writing utensil, and ruler. Have them measure 30″x5″x3”. Show them how to draw the rectangle in a 3-D manner.

-Task a group of 3 of the oldest children to instead make a 3D model of the ark using those dimensions. They will need scotch tape and scissors as well as the other items. A parent may need to assist them.

-Have the group show the tiny scale model of the ark. Ask, “Does it look like the ark you usually see in children’s books? What shape was it really?” (No. A rectangular box.)

YOU WILL NEED: Bible (or Bible app on phone), tape, 3 pairs of scissors, paper (3 sheets taped together), writing utensils, & rulers