Floating and Sinking

How is it possible that huge ships made of steel can float easily in water? Yet in a few hours the same ship can become a sunken wreck like the Titanic. Why does most of an iceberg lie hidden beneath the surface of the water? To answer these questions, you need to find out what makes an object float and what make an object sink.

Comparing Densities One reason objects float or sink is their density. An object that is more dense than the fluid in which it is immersed sinks. An object that is less dense than the fluid in which it is immersed floats to the surface. If the density of an object is equal to the density of the fluid in which it is immersed, the object neither rises nor sinks in the fluid; it floats at a constant level.

Floating and Sinking Now you know why lead sinks: it is several times denser than water. Cork, which is less dense than water, floats. Is an ice cube more or less dense than water?

Iceberg straight ahead! An ice cube floats in water because the density of ice is less than the density of water. But it’s just a little less! So most of a floating ice cube is below the surface. Since an iceberg is really a very large ice cube, the part that you see above water is only a small fraction of the entire iceberg. This is one reason why icebergs are so dangerous to ships.

Buoyant Force If you have ever picked up an object under water, you know that it seems lighter in water than in air. Water exerts a force called buoyant force that acts on a submerged object. Buoyant force acts in the upward direction, against the force of gravity, so it makes an object feel lighter.

Weight vs. Buoyant Force There is always a downward force on a submerged object. That force is the weight of the object. If the weight of the object is greater than the buoyant force, the object will sink. If the weight of the object is less than the buoyant force, the object will begin to float What can you infer about the weight and buoyant force from the last picture?

Archimedes’ Principle You know that Archimedes discovered that a submerged object displaces, or takes the place of, a volume of fluid equal to its own volume. Using this idea, Archimedes came up with a principle that relates the amount of fluid a submerged object displaces to the buoyant force on the object. It states that the buoyant force on an object is equal to the weight of the fluid displaced by the object.

Therefore, the more fluid an object displaces, the more buoyant force it will have. The more surface area an object occupies, the more water is displaces. This is why it is easier to float in water when you are lying down on your back – you take up more surface area, displacing more water, therefore creating a greater buoyant force.

Floating by changing mass Changing the density of an object can make it float or sink in a given fluid. Even though you cannot change the density of an individual substance, you can change the overall density of things that are made of several materials. Submarines can change their overall mass, and therefore their density to allow them to float and sink. They do this by filling or releasing water from their flotation tanks.

When water is pumped out of its flotation tanks, the overall mass of the submarine decreases. Since the volume remains the same, its density decreases when its mass decreases. So the submarine will float to the surface. What do you think a submarine does in order to sink? To dive, the submarine takes in water. In this way, it increases its mass (and thus its density) and sinks!

Most ships are also designed to alter their overall density. The hull (bottom part) of a ship contains a large volume of air. This air reduces the ship’s overall mass, and therefore its density, and helps it to float.

Floating by changing volume Another way of changing density is to change volume. If the mass is kept the same, increasing the volume of a substance, will decrease its density. Increasing the surface area of a substance, will also increase its buoyant force. This is because the greater the volume of water displaced, the greater the buoyant force (Archimedes Principle). The shape of a ship causes it to displace a greater volume of water than a solid piece of steel of the same mass because it has a greater surface area. The more water it displaces, the easier it will float! A ship stays afloat as long as the buoyant force is greater than its weight.

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