# How does a ship float on water?

The science behind floating was first studied by an ancient Greek scientist named Archimedes. He figured out that when an object is placed in water, it pushes enough water out of the way to make room for itself.

Have you ever experienced displacement? Of course, you have! Remember the last time you got into the bathtub and the water level went up?  . When you got into the tub, water got out of your way to make room for you, so the water level in the tub got higher.

When an object enters water, two forces act upon it. There’s a downward force (gravity) that’s determined by the object’s weight. There’s also an upward force (buoyancy) that’s determined by the weight of the water displaced by the object.

An object will float if the gravitational (downward) force is less than the buoyancy(upward) force. So, in other words, an object will float if it weighs less than the amount of water it displaces.

This explains why a rock will sink while a huge boat will float. The rock is heavy, but it displaces only a little water. It sinks because its weight is greater than the weight of the small amount of water it displaces.

A huge boat, on the other hand, will float because, even though it weighs a lot, it displaces a huge amount of water that weighs even more. Plus, boats are designed specifically so that they will displace enough water to assure that they’ll float easily.

## What are boats?

Not such a silly question! A ship or a boat (we’ll call them all boats from now on) is a vehicle that can float and move on the ocean, a river, or some other watery place, either through its own power or using power from the elements (wind, waves, or Sun). Most boats move partly through and partly above water but some (not ably hover craft and hydrofoils) lift up and speed over it while others (submarines and submersibles, which are small submarines) go entirely under it. These sound like quite pedantic distinctions, but they turn out to be very important—as we’ll see in a moment.

You can’t walk onwater: you’re too heavy and you’ll sink like a stone. But this aircraft carrier can float, even though it’s over 300m (1000ft) long, at least a million times heavier than you are, and carries about 70 airplanes and 4000 sailors. Ships (large oceangoing vessels) and boats (smaller ones) are a brilliant example of how science can be put to work to solve a simple problem. Over two thirds of Earth’s surface is covered in water so it’s just as well that science helps us take to the waves. How exactly do ships do their stuff? Let’s take a closer look!

## Why do boats float?

All boats can float, but floating is more complex and confusing than it sounds and it’s best discussed through a scientific concept called buoyancy, which is the force that causes floating. Any object will either float or sink in water depending on its density(how much a certain volume of it weighs). If it’s more dense than water, it will usually sink; if it’s less dense, it will float. It doesn’t matter how big or small the object is: agoldring will sink in water, while a piece of plastic as big as a football field will float. The basic rule is that an object will sink if it weighs more than exactly the same volume of water. But that doesn’t really explain why an aircraft carrier (made from dense metal) can float, so let’s explore a bit further

### Positive, negative, and neutral buoyancy

Buoyancy is easiest to understand thinking about a submarine. It has diving planes (fins mounted on the side) and ballast tanks that it can fill with water or air to make it rise or fall as it needs to.

If its tanks are completely filled with air, it’s said to be positively buoyant: the tanks weigh less than an equal volume of water and make the sub float on the surface. If the tanks are partly filled with air, it’s possible to make the submarine float at some middle depth of the water without either rising up or sinking down.

That’s called neutral buoyancy. The other option is to fill the tanks completely with water. In that case, the submarine is negatively buoyant, which means it sinks to the seabed. Find out more about how submarines rise and fall.

#### I am scientist

I am scientist is a result of small effort to make huge impact on children. To stimulate the curiosity of young scientist towards science. Each idea will start with a experiment followed by reasoning the result , scientific learning, vocabulary (recap the big words used), useful reference website and books, along with free downloadable worksheets.