If you drop a nail into a cup of water it sinks, right? The nail sinks because the density of the steel is greater than the density of the water. But ocean liners, thousands of times heavier than a nail, are made of steel so why do they float, and since they do float, why do they sometimes also sink?
A typical boat
Everyone’s heard of gravity, that mysterious force that pulls everything downward, but did you know there is also an opposing force? Buoyancy is the push to gravity’s pull. Buoyancy is the force that floats your boat. To understand buoyant force, think about what happens when you put an ice cube into your glass of water.
As the ice cube displaces, or pushes away, some of the water it causes the level in the glass to rise and your ice cube to float partially in and partially out of the water like a miniature iceberg. Like a game of tug-of-war; gravity is pulling the ice cube down and buoyant force is pushing it up. How far in or out of the water your ice cube rests depends on its density, or solidness, because that is what the pushing and pulling forces are working against.
A Greek mathematician and inventor named Archimedes noticed that when he stepped into his bathtub the water level rose. He reasoned that the weight of the water he displaced was equal to the buoyant force in the water.
i.e,
Weight of liquid displaced = Buoyant force
This is is called Archimedes Principle and it applies to all fluids.
Ocean liners are made of steel and they float because their density is less than that of the water they float in. The ability of a boat to float depends on its average density. Average density takes into account not just the weight of the steel hull but also the air trapped in it. A ship with a large volume of trapped air has a lower density than that of the water it sits in – so it floats. But there are no air pockets in a nail. Its average density is the same as the density of iron; so it sinks.
When a ships hold is full of cargo it floats lower in the water because there is less trapped air making it denser. Likewise a ship empty of cargo has more trapped air so it’s less dense and it floats higher out of the surrounding water. If the ship loses enough of its capacity to hold trapped air, it sinks. Remember the story about the Titanic?
Example
Consider an iron bar of 20 Kg with 10cm length, 10cm width and 10cm height.
Volume of the bar = 10 x 10 x10 = 1000 cm3
Weight = 20 Kg = 20000 gram
Density of the iron bar = Weight / Volume = 20,000/( 10x10x10) = 20 gram/cm3
The same iron bar is constructed as a ship with air volume 25,000 cm3
Now the total volume of the ship = Volume of the iron bar + Volume of the air space in the ship = 25,000+(10x10x10)
Density of the ship with the same iron bar = Weight / volume
= 20,000 / {25,000+(10x10x10) }= 0.7692 gram/cm3
So, Density decreases when there is more air space. So the average density of the ship as a whole is less than the density of water. Therefore immersed portion of the ship displaces water equal to its weight.
A submarine is designed to have almost the same density of the ocean water that surrounds it. In order for a submarine to sink the crew pumps water into tanks called ballast tanks. To surface the submarine forces the water out of the ballast tanks and fills them with air, lessening the density of the sub and bringing it up to the surface.
Gravity, buoyancy and density work together to determine what floats your boat, or sinks your ship.
Scitech Labs 