Figure 12 The exposed green and red stripes on the ship's hull indicate that the ship is riding high in the water.

Applying Concepts How will the ship's level in the water change if it takes on additional cargo?

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The ability of a huge steel ship to float in water can also be explained in terms of density. As you know, a solid piece of steel sinks in water. A ship's shape, however, allows it to displace a very large volume of water relative to its weight. That is, the ship's shape increases its volume and decreases its density. As long as the ship's effective density is less than that of water, the ship floats.

The heavier the ship, the more water it must displace in order to float. The relationship between weight and the level a ship rides at in the water can be seen in Figure 12. If the cargo ship were completely loaded, it would need to displace more water in order to float.

Why do some balloons float in air whereas others do not? The answer has to do with density differences. Helium and hot air are both less dense than normal-temperature air. When a balloon is filled with either helium or hot air, a buoyant force from the displaced normal-temperature air acts on the balloon. If the size of the buoyant force is large enough, the balloon rises into the air.

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