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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 91 and look at Figure 13.
As you can see in Figure 13, 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 14 A submerged object displaces, or takes the place of, a volume of fluid equal to its own volume. Look at Figure 14. 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
Archimedes’ principle relates the amount of fluid a submerged object displaces to the buoyant force on the object. This relationship is named for its discoverer, 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 Remember that 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 net force on a submerged object will be downward and the object will sink. If the weight of the object is less than the buoyant force, the object will begin to sink. It will only sink deep enough to displace the volume of fluid with a weight equal to its own. At that level, it will stop sinking deeper, and will float. If the 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

Examples The density of lead = 11.3g = 11.3g/cm 1cm
The density of cork is .25 g/cm You could say that lead is more dense than cork. The density of water is 1.0 g/cm so it is less dense than lead but more dense than cork.

Figure 15 By comparing densities, you can explain the behavior of the objects shown in Figure 15. 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 16 Figure 16 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.

Gasses 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.