Floating
How long would you like to stay on a desert island? The chances are you would want to set sail for the mainland soon, especially if your food supply was running low. How would you make your escape? It would depend on the materials you could find. Let's imagine a walk along the beach to find materials to make a raft.

The first thing you find is a sheet of metal. When you throw it into the sea it sinks beneath the waves. Further on you find some metal tanks. When you put them on the water they float, so you drag them back up the shore to start your material collection. After a while you have gathered a collection of planks, logs, rope and twine. You start arranging the tanks and planks on the beach then bind them together to make a raft.

You gather a few items of clothing and food, place them on the raft and push it out into the surf. At first the raft drags along the sand and a wave washes over the front. You fear that your raft will sink but moments later you feel the water carry it and are quite surprised to see how high it floats in the water. You scramble on board and use a paddle to move the raft away from the shore. Once clear of the shore you raise a pole, which serves as a mast, and fix a large sheet of plastic to it to serve as a sail. The wind catches the sail and you are on your way.

What happens when you put an object in water? The object pushes some of the water out of the way to make space for itself. This results in the water that has been pushed out of the way pushing back on the object. The name of the force that the water exerts on an object is called the upthrust. When an object enters the water its weight pushes down and the upthrust pushes back. If the weight is greater than the upthrust the object sinks but if the upthrust is greater than the weight the object floats. You were lucky in your raft building - the weight of the raft was less than the upthrust so the raft floated and you could make your escape.

All substances - solids, liquids and gases - are made from particles. The weight of an object depends on the size of the particles from which it is made and how tightly the particles are packed together. The name given to the way particles pack together in a substance is the density. For example, an object which has large particles, packed tightly together, is said to have a high density while an object which has small particles, packed less closely together, is said to have a low density.

When an object is immersed in water, it pushes water out of the way to make way for itself. If the object has a high density, it has a large number of particles pushing down and this creates a large weight. If the object has a low density, it has a small number of particles pushing down and this creates a low weight. Water is a substance called a liquid. This is made from particles which are not tightly packed that can slide over each other. This packing of the particles gives the water a density too. The upthrust with which the water pushes back on the object depends both on the water's density and the amount that was pushed out of the way.

Remember when you were searching for raft materials and found that the sheet of metal sank. Here is the reason it sank. The metal was made from particles which were packed together closely to make a material with a high density. When it was immersed in the water, it pushed away an amount of water which was equal to its size. The particles in the water were packed less closely than the particles in the metal so the water had a lower density. This meant that the upthrust pushing upwards was weaker than the weight pushing down so the sheet of metal sank.

It may seem strange to find that when the metal is shaped to make a container like a tank it floats. This is the reason: The tank is made from metal sheets, which contain some air. Air is a gas. The particles in a gas can spread out and the gas has a much lower density than a solid. The particles in the metal of the tank and the air have a combined density, which is less than the density of water. When the tank is placed in water only part of it is submerged. The rest is pushed out above the surface of the water by the upthrust which makes the tank float.

By now you can see land and shortly you bring the raft onto the shore close to a village. Your sea journey is over. It has been made possible by the force of the wind on the sail and the power of the upthrust beneath your raft.

What would happen if the tanks had been loaded with food?
Foods are much denser than air so the combined density of the metal and the food would have produced a much stronger weight pushing downwards. However there would have been some air spaces between the items of food and this would have reduced the overall density so the weight would probably be less than the upthrust. As a result the raft would have floated lower in the water. There would then have been a danger of waves breaking over the raft and filling up the air spaces with water. This would increase the weight of the raft still further and possibly cause it to sink.

Would the raft float just as well in a river as on the sea?
No. The raft would float higher in the sea than it would in a river. The reason for this is that seawater contains a large amount of salt and river water does not. The large amount of salt makes the seawater denser than the river water. As the upthrust depends on the density of the water, the water with the higher density has the stronger upthrust and pushes the raft higher out of the water.

Can the different densities of water be a danger to shipping?
Yes, it can. Saltiness and temperature affect the density of water. For example, sea water in the North Atlantic is very cold. This makes the water very dense and gives it a strong upthrust on any ship passing through it. Water in a tropical river is formed from warm freshwater. This is much less dense than the water in a cold sea and gives a weaker upthrust to a ship passing through it. Imagine a ship, fully loaded so that it floats in a cold sea, passing into a tropical river. The weakness of the upthrust would make the ship sink lower in the water and possibly might make it sink completely!

Is there a way ships can be loaded safely?
Yes, there is. On the side of every ship are Plimsoll lines. They show the level to which a fully loaded ship will sink in the different waters of the world. When the ship is loaded to the correct Plimsoll line it can travel anywhere without its weight being greater than its upthrust. The lines are named after Samuel Plimsoll who introduced the idea of using safe loading lines on ships.

If a ship is loaded to its correct Plimsoll line will it never sink?
The ship might sink in a very severe storm because waves might break over it and enter air spaces in the ship's hull. The extra water then increases the ship's density and weight. If the upthrust cannot match the weight of the ship plus the water it has taken on board, the ship will sink.

Can one liquid float on another?
Yes, it can. If two liquids with different densities are mixed together the less dense liquid will float on the denser liquid.

What makes a party balloon rise in the air?
A party balloon is filled with a gas called helium. This gas has much smaller particles than the particles of the air. This means that the density of the helium in the balloon is much less than the density of the air around it. The helium balloon pushes air out of the way just like an object immersed in water pushes water out of the way. The air that has been pushed out of the way by the balloon pushes backward on the balloon with a force called the upthrust. This force is larger than the weight of the balloon, so the balloon is pushed upwards until it is stopped by the ceiling.

Why do party balloons eventually sink?
There are tiny holes in the material which is used to make the helium balloon. They prevent air particles entering the balloon but are not small enough to prevent helium particles escaping into the air. After a few days the helium balloon sinks and shrinks because a large number of helium particles have escaped.