Floating
You may be wondering why a piece of steel sinks, whereas a huge steel ship floats. A heavy steel ship floats because of the shape of its hull. The hull is shaped so that it displaces a large volume of water, creating a large buoyant force. The buoyant force created by the ship's hull is large enough to counteract the ship's tremendous weight.
Objects also float more easily in dense fluids. Why is this? As you know, the buoyant force acting on the object equals the weight of the volume of the fluid it displaces. For a given displacement, the denser the fluid is, the greater is the weight displaced. This greater displaced weight results in a greater buoyant force. This is why it is easier for a person to float in very salty water. The dense salty water produces a larger buoyant force when displaced by the person's body.
B Neutral buoyancy Once the submarine's ballast tanks hold enough water for the submarine's weight to equal the buoyant force, the submarine is said to have neutral buoyancy. It now has no tendency to rise or sink.
C Positive buoyancy To rise again, the submarine's weight must be decreased. Air is pumped into the ballast tanks to force the water out. The buoyant force acting on the submarine now exceeds the submarine's weight. With positive buoyancy, the submarine rises.
Table of Contents