Floating and Sinking Think about this… How is possible for a huge ocean liner like the Titanic to float? How is possible in a few hours for it to become a sunken wreck?

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

Take a look… Turn to page 381 As you can see in Figures 7 & 8, a fluid exerts pressure on all surfaces of the submerged object. Since the pressure in a fluid increases with depth, the upward pressure on the bottom of the object is greater than the downward pressure on the top. The result is a net force acting upward on the submerged object. This is the buoyant force.

Figure 9 A submerged object displaces, or takes the place of, a volume of fluid equal to its own volume. Look at Figure 9. An object that floats on the surface of a fluid, however displaces a smaller volume. It displaces a volume of fluid equal to the portion of the object that is submerged.

Archimedes’ principle relates the amount of fluid a submerged object displaces to the buoyant force on the object. the ancient Greek mathematician Archimedes. Archimedes principle states that the buoyant force on an object is equal to the weight of the fluid displaced by the object.

Floating and Sinking There is always a downward force on a submerged object. That force is the weight of the object. Weight of object > buoyant force = downward force and object will sink!

Floating and Sinking Weight of object < buoyant force= object will begin to sink. Only sink deep enough to displace the volume of fluid with a weight equal to its own. It will stop sinking deeper, and will float. Weight of the object is exactly equal to the buoyant force, the two forces are balanced.

Density Exactly why do some objects float and other sink? By comparing density of an object to the density of a fluid, you can decide if it will float. What is density? – The density of a substance is its mass per unit volume. Density = Mass Volume

Figure 11 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. And 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. Instead it floats at a constant level.

Figure 11 Figure 11 also shows several substances and their densities. Notice that liquids can float on top of other liquids. You may have seen that salad oil floats on top of vinegar. Objects with the greatest densities are near the bottom of the cylinder.

Gases Don’t forget that air is also a fluid. Objects float in air if their densities are less than the density of air. A helium balloon rises because helium is less dense than air. An ordinary balloon filled with air, however is more dense than the surrounding air because it is under pressure. So the balloon falls to the ground once you let go of it. Changing the density of an object can make it float or sink in a given fluid.

Submarines The density of a submarine, for example, is decreased when water is pumped out of its flotation tanks. The overall mass of the submarine decreases. So the submarine will float to the surface. To dive, the submarine takes in water. In this way it increases its mass and density and sinks.

Buoyancy and Density Another way to change density is to change volume. In figure 17, the amount of steel present in the three objects is the same. Yet two of the figures float, and one sinks. Solid steel sinks rapidly in water, and so will the hull of a ship that is full of water. Usually however, the hull of a ship contains a large volume of air. This air reduces the ship’s overall density and so allows it to float.