Buoyancy ( It's What Floats Your Boat )

Buoyancy can be a difficult concept to grasp.

Is it a vector* force?

Is it pressure* differences?

How does gravity* come into play?

Let's not get caught up in the physics of it all. Let's take a hint from the last question and PLAY with buoyancy to learn about it.

Hopefully with the following activities Buoyancy will become a much easier concept to understand.

This Instructable consists of 3 different yet related activities all about buoyancy:

A. Water Bottle Boat (The Water pushed aside by the hull weighs the same as the cargo?)

B. Cartesian Diver (How could we not mention this simple buoyancy demonstrator?)

C. Three Balloons (Gases are fluids? Buoyancy works in these fluids too.)

All activities should be able to be performed with household items, and a trip to the hardware and big box store.

All these activities focus on simple Buoyancy defined by Archimedes' Principle:

"The magnitude of the buoyant force on an object is equal to the weight of the fluid it displaces."

Come have fun exploring Buoyancy --

*If you want to get really technical about buoyancy you can take a look at the Physics Hypertextbook. [ I try to avoid this kind of stuff if at all possible ;-) ]

Supplies

A. Water Bottle Boat

500ml empty water bottle

A Bunch (technical term) of Small Stones (Dollar Store has these)

Wood Board 3" x 16" x 1/4"

2 Disposable Storage Containers

Round Pencil (not hexagonal)

Marking Pen

Utility Knife

Glue Gun

Optional - Boat Flag (construction paper)

B. Cartesian Diver

2 Liter Plastic Soda Bottle

Drink Straw (Clear not required but helps visualization)

1/4" - 20 Steel Hex Nut (Stainless steel will help resist rust)

Hot Glue

C. Three Balloons in a Fluid

Three Balloons (each a different color)

Electronic Spray Duster ('Canned Air'; Difluoroethane)

Helium (Party Balloon Kit works)

Optional - Digital Scale

Step 1: Archimedes' Principle Is SIMPLE

Archimedes of Syracuse (c. 287 – c. 212 BC) was a Greek mathematician, physicist, engineer, inventor, and astronomer. He is regarded as one of the leading scientists in classical antiquity.

In other words he lived a long time ago and was really really smart.

The Archimedes' Principle:

"The magnitude of the buoyant force on an object is equal to the weight of the fluid it displaces."

What this really means is that when an item floats it does so because the material (water, gas, etc.) pushed out of the way (displaced) by the object weighs the same or more than the object.

Step 2: A. WATER BOTTLE BOAT - Build a Balance

A. WATER BOTTLE BOAT

Build a Boat? Not so fast...

In order to test Archimedes' Principle:

"The magnitude of the buoyant force on an object is equal to the weight of the fluid it displaces."

We need a balance to see if the cargo of the boat is heavier than the weight of the water it pushes out of the way.

We can make a simple balance by taking a board and hot gluing down two disposable food containers - one on each end.

Then take a round pencil and cut it about a 1/2 inch longer than the board is wide, so 1/4 inch extends past each side of the board.

Place the pencil under the board in the center and twist it to roll it toward one end of the board or the other to get the board to balance or be nearly in balance as possible.

Tack the pencil into place with some hot glue. After the glue cools flip the board over and hot glue the length of the pencil to the board to fasten it securely.

You now have a balance that will let you compare the weight of items to know which is heavier or lighter.

Step 3: WATER BOTTLE BOAT - Build the Boat

Now we can build a boat.

Our Water Bottle Boat is made from a water bottle - who knew?!

Cut the opening of our boat out of the side of the water bottle.

Cut the sides down no farther than to the midpoint of the side of the bottle.

If you want you can hot glue the remainder of the pencil to the boat to be a mast for a construction paper flag!

Float your boat in a body of water (sink, tub, pool, etc.). To keep it stable you will need to add a few rocks to start.

Load rocks into the boat evenly until you have a reasonable cargo. You probably want about a 1/2 to 1/4 inch of 'hull' sitting above the waterline.

Step 4: WATER BOTTLE BOAT - Weigh the Water

Take a marking pen and carefully trace the waterline on the inside of the bottle. This is done so that we can determine how much water is pushed aside (displaced) by the boat.

Remove the boat from the water and place the rocks into one side of the balance.

Fill the boat with water up to the waterline marking you made. This is an estimate of the amount of water the boat's hull pushes aside (displaces).

Pour the water into the opposite side of the balance.

The water should equal or weigh more than the cargo of rocks and the weight of the boat too!

The balance tips toward the water showing that the water weighs more than the rocks and the boat -- That's why the boat floats -- the boat and it's cargo weighed less than the water the boat pushed aside.

[Obviously this is an imperfect experiment with lots of error introduced along the way - but it works.]

Step 5: B. CARTESIAN DIVER

B. CARTESIAN DIVER

The Cartesian Diver is named after René Descartes (31 March 1596 – 11 February 1650) who was a French philosopher, mathematician, and scientist, as it is said that he invented the toy. Toy? It demonstrates Archimedes' buoyancy principle. Maybe it should be considered much more than a toy.

There are many ways to make a Cartesian Diver. I tried to use common materials that could be obtained locally so that it would not require any part to be ordered.

The weight of the hex nut keeps the capped straw upright with air trapped inside. The straw will be contained in a sealed 2 liter plastic bottle. The air in the straw displaces enough water to make the straw float to the top of the bottle.

After the bottle is sealed, squeezing the bottle compresses the air in the straw, making it take up less volume and thereby displacing less water and the straw becomes less buoyant and sinks.

To make your Cartesian Diver:

Cut a plastic drinking straw down to 5-1/2 inch in length.

Use hot glue to seal up the end of the straw. Use just enough to cap the straw so it will hold air.

Crease the open end of the straw so it will fit into the hex nut.

Pull the nut up the straw until it is about an inch from the open end of the straw.

Fill the 2 liter bottle with water. Place the 'diver' (straw) in the water and confirm that it just floats.

If it floats to high you can reduce the straw length. If it sinks, you will need to cut a longer straw to trap more air.

After the straw just floats cap the bottle and give it a squeeze to test your diver.

Squeezing should make the 'diver' fall and releasing should have the 'diver' float to the top of the bottle.

Squeezing reduces the volume of air in the straw so that the water displaced by the air, hex nut and straw weighs less than the hex nut and straw, so the 'diver' will not float, but sinks.

Step 6: C. THREE BALLOONS in a FLUID - Fill the Balloons

C. THREE BALLOONS in a FLUID

The water that the boat was floating in is a fluid. But gases, including the air that is all around us, are also considered fluids.

Balloons filled with gases also act according to Archimedes' Principle. If the fluid an object displaces weighs more than the object, it will float. The opposite is true, if the fluid an object displaces weighs less than the object the object will sink.

Balloons can be filled with gas that is lighter (less dense) than air, for example, helium. Or a balloon can be filled with a gas that is heavier (more dense) than air, such as 'canned air'/electronic duster (Difluoroethane).

Do not breath the gases used in any of the balloons. Doing so can be life threatening.

The can of 'canned air' (Difluoroethane) I used states: Keep out of the reach of children. Deliberately inhaling can be fatal. Use in well ventilated area. Flammable. Do not use near hot surfaces or ignition sources.

Fill each balloon with the following:

1. Helium - Lighter than air

2. Air Mix - Mixture of about 2/3 Helium and 1/3 Air. Trying to be neutrally buoyant. Neither sink nor float. The helium overcomes the weight of the balloon.

3. 'Canned Air' electronic dusting spray; Difluoroethane

(I suspect that most gases used in 'dusters' are heavier than air so it may not be critical that the duster contains difluoroethane.)

Step 7: THREE BALLOONS IN a FLUID - Release the Balloons

Carefully hold the balloons together so they can be released simultaneously. Use your finger tips to make sure that your hands will not stick to the balloons.

Release the balloons by slowly moving your hands away without creating much air movement.

See what happens.

The air displaced by the helium filled balloon weighed more than the helium in the balloon and the balloon floated upward.

The air displaced by the air/helium mixture balloon weighed almost the same as the mixture inside the balloon so the balloon wanted to stay where it was an neither raised or fell.

The air displaced by the difluoroethane balloon did not weigh more than the difluoroethane in the balloon and so the balloon sank.

A heavy balloon? It is a very strange sensation holding a balloon filled with difluoroethane. It is perceivably heavy - strangely so. If you try to toss it around it quickly falls to the ground.

The scale shows that a balloon full of difluoroethane weighs about 10 grams and the empty balloon weighs about 2 grams. So the difluoroethane in the balloon weighed about 8 grams! No wonder holding it feels so strange.

Step 8: NEGATIVE BUOYANCY

Negative Buoyancy is where the object in the fluid weighs more than the fluid it displaces. The difluoroethane weighed more than the air it displaced so it sank. The same can be true for our water bottle boat.

Now you can have your boat be negatively buoyant; want to sink instead of float.

Just pile in more rocks until the water rushes over the sides and down to Davy Jones' Locker she goes. The boat and rocks weigh more than the water the boat can displace.

You can weigh the boat and rocks vs water displaced and see that the rocks and boat weigh more than the water they displaced.

Positive Buoyancy and Negative Buoyancy - it comes down to which weighs more: the object or the fluid it displaces.

Hope you had fun learning about buoyancy!

Enjoy!