Boats float primarily because of a principle known as buoyancy.
The Science of Buoyancy
Buoyancy is the upward force exerted by a fluid (like water) that opposes the weight of an immersed object. For an object to float, the upward buoyant force must be equal to or greater than the downward force of its own weight.
How Displacement Works
When a boat is placed in water, it pushes aside or displaces a certain volume of that water. The key factor determining whether the boat floats is the weight of this displaced water compared to the weight of the boat itself.
According to the principle, a boat floats when it has displaced just enough water to equal its own original weight – a principle called buoyancy. The water exerts an upward force equal to the weight of the water that has been displaced.
Floating Heavy Objects
This is why even very heavy boats or ships can float. The boat is shaped in a way that allows it to displace a large volume of water.
- Example: A large ship made of heavy steel and carrying cargo is very heavy. However, the ship's hull is designed to be wide and deep below the waterline. This design allows it to push away a vast amount of water.
The reference states, "Even though a ship is very big and very, very heavy, it is not as heavy as the water it pushes away." This means the weight of the huge volume of water displaced by the ship is greater than the ship's own weight, resulting in a strong upward buoyant force that keeps it afloat.
In simple terms:
- A boat sinks until it has pushed aside water that weighs exactly the same as the boat.
- Once the weight of the displaced water equals the boat's weight, the upward buoyant force balances the downward gravitational force, and the boat floats.
The shape of a boat, particularly its hull, is crucial because it allows it to displace a significant volume of water, even if the materials it's made of are denser than water. A solid piece of steel, for instance, sinks because it cannot displace enough water to equal its own weight; its volume is too small for its mass. But shape that same steel into a boat hull, and it can displace a much larger volume of water relative to its weight.
