Any object, wholly or partially immersed in a fluid, is buoyed up by a force equal to the weight of the fluid displaced by the object.

Archimedes of Syracuse

Read the above statement to any class, and you will be greeted with a sea of blank faces. Tell the story of the gold crown, the crooked smith, the bathtub and the streaking scientist, and you will get some interest and some snickers. Hold up a beach ball, and ask students to show with their hands what happens if the ball is pushed down into the water. Faces around the room will light up as hands fly into the air. But all three approaches teach the same basic concepts of buoyancy.

Ask any kid what makes things sink or float, and s/he will say something to the effect of “heavy things sink; light things float”. The reality is a bit more subtle; heavy things are pulled down by gravity more than light things, but large things are buoyed up in water more than small things. The density of a substance is mass per unit volume, or how heavy divided by how large, and densities of common substances are listed in Table 1 for the instructor’s use. However, it is not clear that quoting the definition of density enhances a student’s understanding of density and buoyancy. What is needed is a real physical sense – students need to feel how different objects behave in water.

The lesson plan described here started as a take-home lab for undergraduates, and is very popular among early primary grade students. I shall describe the kids’ version of the lab, and note upgrades for older students as appropriate.

Floating Soda Bottles

The basic setup is good for 4-5 students; scale up to your classroom as appropriate. Use an un-carpeted room, such as an art room or cafeteria. You will need:

  • one large tub half full of water – e.g.a large ice bucket intended for parties
  • one large empty plastic bottle with lid –e.g. a 2-liter soda bottles
  • 2-3 identical medium-sized plastic bottles with lids – e.g. 16 or 20 oz soda bottles
  • 1-2 small plastic bottles with lids – e.g. an 8-oz water bottle
  • enough sand to fill a medium sized bottles
  • enough pennies to fill a small bottle
  • old bath towels for clean up

What makes things float? Have the kids take turn pushing the empty bottles down into the water, and ask them to describe what they feel. Ask them to compare what they feel with the different size bottles. Have them step away from the water tubs for discussion. Solicit their various descriptions of the experience; most will say something similar to “the bottles push back and the big bottle pushes back the most.” Ask why the big bottle pushed back the most. Some students will claim that the larger bottle is heavier – have them weigh a large and medium empty bottle in their hands. Note that you do not actually have to use the word buoyancy for younger kids, but can introduce the concept for older students.

Do the floating bottle dance, and have everyone join in the gestures. Have the kids say the words on the second run through. Repeat as needed. “When you push the soda bottle down into the water (push down with both hands), the bottle pushes the water out of the way (both hands out to the side). The water pushes back (both hands toward the middle) and makes the bottle float (both hands facing up, bounce a bit). Ask the kids which bottle pushes the most water out of the way, and which bottle gets pushed back hardest. I have definitely seen the lightbulbs go on over kids’ heads at this stage.

What makes things sink? Have students fill one of their medium sized bottles completely with water (no air bubbles). Have students compare the full and empty bottles of the same size. Which one floats? Which one sinks? Which one weighs more? Does the full bottle sink all the way to the bottom or just float near the surface? What would it take to get it to sink all the way to the bottom? Explain that the water inside the bottle weighs as much as the water pushed aside by the bottle, so the downward and upward forces are equal.

What if the bottle were full of sand? What if it were half full of sand? Demonstrate with a bottle half full of dry sand – if laid on its side, the bottle will float at the surface. Standing up, it may sit on the bottom, with the top sticking out, but that is not the same as sinking. Why doesn’t the half full bottle sink? Review the idea that the bottle’s volume determines how much it is pushed up by the water. Allow students to predict and observe how much sand needs to go into the bottle to make it sink to the bottom. Repeat the experiment using copper pennies, which are significantly denser than sand (Table 1). Note that a pile of pennies will have gaps, and be less dense than a single penny. To sink a soda bottle, you must fill it at least ¾ with sand, or about half full with pennies.

How do forces balance? For this experiment, be sure to compare bottles of the same volume. One bottle should be completely full of water (no air bubbles) and the other should contain only air. One student should hold the full bottle in his/her left hand (palm up, at shoulder height) and, at the same time, push the empty bottle down into the water with his/her right hand. The full bottle weighs 20 oz, and the empty bottle is pushing 20 oz of water aside. The effort required to hold up the full bottle should be similar to the effort required to push the empty bottle down. Teachers tend to perceive the efforts as similar; students tend to think one effort is harder than the other. Remind students that some of the forces they’ve been measuring are a lot different from each other; ask whether the lifting up and pushing down efforts are a lot different or just a little different.Older students may be challenged to measure the forces more precisely than can be done by hand.

How dense are other substances? You may wish to explore the density of oil or of salt water, by layering these fluids with fresh water. However, compared to fresh water, ocean water only 3% denser than fresh water, and vegetable oil is only 8% less dense. Soda bottles filled with these fluids will float at slightly different depths, but the effect is quite subtle, and can easily be obscured by air bubbles in the bottles. Also, the plastic in the bottles is significantly denser than ocean water, and may affect your measurements.

Flinking Plankton

The experiments described above can be used on their own, or as part of a physics class. Being a marine scientist, I like to introduce and justify the study of buoyancy by discussing the importance of phytoplankton to our lives, and having the kids make model “flinking” plankton. For this part of the class, in addition to your tubs of water, you need:

  • large photos of phytoplankton
  • at least one wine cork per child
  • lots of push pins and small nuts or washers
  • nails are hard for small hands to push into corks – not recommended

What’s so great about plankton?A variant of this script may suit your teaching style. “Everybody take a deep breath … ahhh. Where does the oxygen in that breath come from?” Students will mention various plants, but especially trees. “Trees are great; everybody be a tree (arms up). It’s easy to be grateful to a tree – it’s big and provides shade as well as oxygen. Hug your tree (hug yourself). But at least half the oxygen in that breath came from the ocean. So let’s go down to the ocean with an eyedropper. We’ll take a tiny drop of water, and place it on a microscope slide (act out the motions). Now focus … mm hmm … do you see the little green specks in all the weird shapes?” Now hold up or pass out your photos, explain that phyto-plankton are tiny floating plants, and that we could not be here without phytoplankton making oxygen.

“Who likes seafood?” Explain that all seafood is supported by the oceanic food chain, which is based on phytoplankton. You may wish to read “This is the Sea that Feeds Us[1]” You may also wish to mention the role of phytoplankton in absorbing CO2 and moderating the climate.

How do plankton survive? Explain that phytoplankton must live just below the surface of the water; close enough to the surface that they get sunshine, but not right on the surface where they can get dried out. Float a cork in your tub to demonstrate that part of the cork sticks out of the water where it can dry out. Drop a washer or nut in your tub to demonstrate that it sinks to the bottom (which in the ocean, is 4000 m down). Ask the students why the washer sank and the cork floated; let them weigh the two items in their hands to understand that the cork is actually heavier. Pin the washer to the cork, and it will still float at the surface.

Challenge the students to make a model plankton out of corks, pins and nuts. It should float below the surface, neither floating to the surface, nor sinking to the bottom, but “flinking”. Alternately, challenge the students to make a model plankton that will sink to the bottom as slowly as possible. This exercise was developed by UC Berkeley’s MARE[2] program, and is available in several forms; Google “great plankton race” for resources.

Students tend to under-estimate how many nuts and pins it takes to sink a cork. Definitely have an example of a flinking plankton on hand to demonstrate. Also suggest that they start by adding enough pins to make the cork sink all the way to the bottom, then remove one pin at a time. Once you have a successful flinker at each tub, you can move on to the soda bottle experiments. As a bridge, remind them that the floating cork was somewhat heavier than the sinking washer, but a lot larger.

Streaking Scientists and the Magic Schoolbus

The Magic Schoolbus video “Ups and Downs” offers a very good demonstration of the effects of mass and volume on buoyancy and could be used with the soda bottle experiments suggested here. The video tells a story of a submarine moving up and down in the water, variously by adding and removing weight, and by changing its size. Ms Frizzle offers a very clear explanation of buoyancy, that I used in my buoyancy dance.

Older students will appreciate the story of Archimedes and the golden crown. The king of Syracuse gave a certain weight of gold to a smith, to be formed into crown. Upon receiving the crown, he suspected that the smith may have kept part of the gold, and filled the crown with silver. The king asked Archimedes how to determine the composition of the crown without destroying it. Archimedes knew that, if the gold crown was diluted with silver, it would have a slightly lower density than pure gold, and a slightly larger volume.

In his bath that evening, Archimedes noticed that his bath overflowed when he sat down, and also that he could feel the bath water lifting him up. He realized that he was being lifted by the weight of the water he displaced, and delighted with his discovery, sprinted down the street, naked and dripping and yelling “Eureka!”.

Archimedes principle allows us to estimate the density of any object. The object is first weighed precisely. Then it is submerged in water in a precisely graded cylinder; the volume in the cylinder will increase with the volume of the object. However, C. Rorres[3] points out that Archimedes would not have had access to precise measures of weight or volume. Instead, Archimedes may have balanced the questionable crown and a lump of pure gold, first in air, then submerged in water. If the gold was diluted, it would be less dense than pure gold. When the crown and gold were submerged, the balance would tilt up toward the crown. This is an excellent experiment to replicate with high school or college students.

Evaluation and Assessment

There is no doubt that these experiments are as lot of fun. Some of my undergraduates still remember the Archimedes experiment, several years after taking my class. Kids love to play with water, and this program has been very popular. As I present this program informally, I have not been able to test kids’ retention of the concepts.

A few key questions will indicate students’ understanding of buoyancy. Given two objects of the same mass, but different volumes, which object is more likely to float? Given two objects of the same volume, but different masses, which is more likely to sink? Ask students to repeat the floating bottle dance. Ask them to explain in their own words why the larger bottle is harder to push down into the water (“it pushes more water away” is correct, “it has more air” is not).

One more thing …

Kids love to take things home. I always end my program by helping each kid make an “ocean in a bottle” to take home. The simplest way to do this is: fill a single serving plastic beverage bottle halfway with colored water and top off with vegetable oil. Try to eliminate air bubbles, and add sparkles and shells if desired. When this bottle is tilted gently, it will form beautiful waves. You can also use the density difference between oil and water to illustrate or test your students’ understanding of buoyancy.

Substance / Density
Vegetable oil / 0.92 gram/cubic centimeter (g/cc)
Fresh water / 1.00 g/cc
Ocean water / 1.027 g/cc
Lego toys / 1.06 g/cc
PET plastic used to make soda bottles / 1.4 g/cc
Loose dry sand / 1.4 g/cc
Solid glass / 2-3 g/cc
One penny / 7.14 g/cc
Silver / 10.5 g/cc
Gold / 19.3 g/cc

[1] This is the Sea that Feeds Us, R. Baldwin and D. Dyen, beautifully written and illustrated

[2] Marine Activities Resources and Education,

[3] a superb resource