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Buoyancy Activities & Experiments

Buoyancy is an object's ability to float in a liquid. Floating is caused by an upward force known as the buoyant force, exerted against the object in the liquid. Buoyant force is a result of the difference in pressure at the top and bottom of the object. The pressure at the bottom must be greater than the pressure at the top for an object to float. Children can learn about buoyancy by conducting simple experiments using common household items.

Float the Boat
- A ship must weigh less than the water that it displaces in order for it to float. The air within the ship helps lower the actual weight of the ship so that it weighs less than the water. To demonstrate this, roll a piece of clay into a ball and drop it into a bucket of water. It will sink because it does not have any air in it. Take another piece of clay and form it into the shape of a small cup or bowl. Form a small keel, or a ridge at the bottom of the cup to act as the spine. Place it in the same bucket of water. It should float because it has air pressing against the top of the cup.
Orange Peel
- Drop a whole orange into a small bucket or a cup of water. Note where the water is before placing the orange in the water and again after the orange has been dropped. The water should rise and the orange should float. Now remove all of the orange peel and place it back in the water. The orange should sink with the peel removed because the peel contains air bubbles that help it weigh less than the water it displaced.
Sink to Float
- Fill a clear cup or glass three-quarters of the way full with 3 parts water and 1 part vinegar. Slowly pour in 1 teaspoon of baking soda so that the mixture does not bubble over. After the bubbles subside, slowly add in 1 more teaspoon of baking soda. Drop in various pieces of food such as small pieces of vegetables and noodles or rice. Watch as the pieces slowly sink to the bottom and then rise back up. This occurs when the objects cling to carbon dioxide bubbles created by the vinegar and baking soda mixture. The bubble carries them to the top until it bursts and the object sinks again.
Changing Density
- Objects will float if the density of the liquid is changed, causing the water that is displaced to weigh more than the object, creating more pressure on the bottom of the object and forcing it upward. Fill a glass halfway with water and place an egg in it. Fill another glass halfway with water and then stir in about 6 tablespoons of salt. Place an egg in the cup and watch as the egg floats. The salt has changed the density of the water.

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