But you already know this because what people often refer to as an object’s “weight” is actually its gravitational force (Fg), and for a given volume, weight relies solely on density. So, if you toss these blocks into a lake, the styrofoam will float while the steel will sink. This clearly relates to density.
Now, consider a block of water that takes up the same volume. If you could somehow hold this cube of water, it would feel pretty heavy—about 62.4 pounds. But when you place it carefully in the lake, will it sink or float like the styrofoam? Neither, right? It’s just going to sit there.
Since the block of water doesn’t move up or down, the total force acting on it must be zero. This means there’s a force pushing up with equal strength to counteract gravity. We call this force buoyancy, and for any object, the buoyancy force equals the weight of the water it displaces.
Let’s break this down. The steel block displaces the same amount of water, meaning it experiences the same upward buoyancy force as the block of water. However, since it’s denser and has more mass, it sinks. Generally, an object will sink if the gravitational force is greater than the buoyancy force, and it will float if the buoyancy force exceeds the gravitational force. In simpler terms, an object sinks if it’s denser than water and floats if it’s less dense.
Then there’s the concept of neutral buoyancy, where an object neither sinks nor rises to the surface. Humans are fairly close to neutral buoyancy since our bodies are 60 percent water. That’s why you feel weightless underwater—the buoyancy force nearly balances out the gravitational force.
Now, wait a minute. Aircraft carriers are made of steel and can weigh 100,000 tons, so why do they float? Can you guess? It’s all about their shape. Unlike a solid block of steel, a ship’s hull is hollow and filled with air, giving it a large volume relative to its weight.
But what happens when you start loading it with cargo? As the ship gets heavier, it must displace more water to achieve that balance. Generally speaking, when you launch a boat or ship into the water, it will sink until the weight of the water it displaces equals the total weight of the boat.
