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By Janice VanCleave
Printed in Practical Homeschooling #65, 2005.

Why a metal boat floats, but a metal ball sinks.
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Janice VanCleave

Fun Time

Purpose. To determine how a heavy ship floats.

Materials. Two 12-inch (30-cm) square pieces of aluminum foil, 20 paper clips, small bucket, tap water


  1. Wrap one of the foil squares around 10 paper clips and squeeze the foil into a tight ball.

  2. Fold the four edges of the second foil square to make a small boat. Place the remaining 10 paper clips in the boat. Spread the clips as evenly as possible.

  3. Fill the bucket with water.

  4. Set the foil boat and ball on the surface of the water in the bucket.


The boat floats and the ball sinks.


The ball and boat both have the same weight, but the ball has a smaller volume than does the boat. The weight of water pushed aside by an object equals the force of water pushing upward on the object. The hollow boat has a larger volume than the compressed ball, thus it displaces more water. This results in a more buoyant force on the boat, causing it to float. Ships are very heavy, but they have a large volume, which increases their buoyancy.

This experiment was taken from Janice VanCleave's Science Through the Ages and used by permission of the publisher, John Wiley & Sons, Inc.

For more information about buoyancy, check out Janice VanCleave's Play and Find Out About Nature (ages 4-7) and Janice VanCleave's Scientists Through the Ages (ages 8-12).

Q: Why do I feel lighter in a swimming pool?

A: Your sense of feeling lighter is because the water is pushing up on your body.

Before the Greek mathematician and inventor Archimedes (c.287-212 B.C.), nobody knew why some things float and others don't. It is said that Archimedes' king gave a silversmith a certain amount of gold to make him a new crown. When the crown was finished, the king asked Archimedes to figure out a way to secretly determine if the crown was made of pure gold or a mix of gold and cheaper metals, as the king suspected. Archimedes was puzzled as to how to do this, until he stepped into a bathtub and observed that his body displaced (pushed out a place) a certain volume (amount of space a material occupies) of water from the tub.

Gold has a specific density, or mass per unit volume. This means that there is a specific amount of gold material in a given volume. Archimedes realized that he could determine the volume of the crown by measuring the volume of water displaced by the crown, and using the volume and the mass of the crown, he could determine its density. It is said that he is so excited about his discovery that he ran named through the street yelling, "Eureka!" (I have found it!). He determined that the density of the crown was not that of pure gold, and the silversmith who made the crown was killed for stealing from the king.

Whether this story is true or not, Archimedes did discover a technique for determining density using water displacement. He also discovered that fluids, such as air or water, apply an upward buoyant force on objects that are partially or totally submerged in the fluids. Such object are said to have buoyancy (the tendency to float when placed in a fluid). The buoyant force is equal to the weight of the fluid that the object displaces. This natural law is called Archimedes' principle.

Whether a material floats or sinks in a fluid doesn't depend on its weight but rather on its density. An object with a density greater than the fluid it is in sinks, while an object with a density less than the fluid floats. This explains why a heavy ship or can float on water. Because the ship is hollow, the weight is spread out and the ship displaces a large volume of water. According to Archimedes' principle, the weight of this displaced water is equal to the buoyancy force pushing up on the ship. If the ship were compressed (pressed together) into a solid block, it would weigh the same but its density would be greater than water and it would sink. The small solid block would displace less water than the large hollow ship and its buoyancy would be less.

When you are in water, your weight (the measure of the force of gravity acting on your body) is pulling you down. But the buoyant force of the water is pushing you up. So you feel lighter. How well you float depends on your body density. The lower your density the more buoyant is your body. You can decrease your body density by taking a big breath of air. This expands your lungs making your volume greater and your density less.

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