John Philip Holland and the Story of Density & Flotation

John Philip Holland came from Liscannor, Co. Clare and is best known for developing the world’s first submarine. Originally a maths teacher, he started to think about how a submarine could work – he worked out the physics involved and designed and tested it successfully. Through the story of  John Philip Holland, we’ll explore the physics of density and flotation.

John Philip Holland was born in the coastal town of Liscannor, Co. Clare in 1841. After finishing school, Holland worked as a teacher for the Christian Brothers in Limerick, Cork, Drogheda and Dundalk before emigrating to the United States in 1873.

While a teacher in Cork, Holland read an account of a battle during the American Civil War. He realised that the best way to attack such ships would be through an attack beneath the waterline. He drew a design, but when he attempted to obtain funding, he was turned away. After his arrival in the United States, Holland slipped and fell on an icy Boston street and broke a leg. While recuperating from the injury in a hospital, he used his time to refine his submarine designs and was encouraged by Isaac Whelan, a priest. He tried a few times to perfect his designs until eventually, in 1897 he created the first submarine having power to run submerged for any considerable distance, and the first to combine electric motors for submerged travel and gasoline engines for use on the surface. Holland figured out that the key to inventing the submarine was to understand the physics of density and flotation.

What is density?

You may be familiar with an objects mass (which is the amount of matter it contains), but mass alone is not the best way to compare between different materials, we also need to take into account the volume. Imagine you are walking in the forest, you put up a tree branch in your right hand and some tree leaves in your left hand. You want both hands to have the same amount of mass in each of them. What do you think you’ll have to do? You’ll probably have to pick up a lot more leaves, to balance out the weight of the branch, right? This means you’ll have a bigger volume of leaves, than the branch…but they’ll both have the same mass. The reason for this is that the leaves and the branch have a different density.

Density is a property of a material that can be used to easily compare between different materials. Density is defined as the mass of an object divided by its volume. (Density = Mass / Volume). Because it combines two different properties of an object, we call it a compound unit. Density is given in the units kg/m3 or g/cm3. If you want to know a material’s density, you just have to measure its mass and its volume. However, if you know its density beforehand, you can measure the volume to find its mass. In this case, Mass = Density x Volume.

Flotation and buoyancy

The most practical application of density is flotation, where a solid object floats in a liquid or gas. Water has a density of 1000 kg/m3 (or 1 g/cm3). Any material that has a density lower than water will float on top of it. When water freezes into ice, its density gets smaller. This is why ice floats on water!


Try at home!

Fill a glass with water, then pour some olive oil into the glass. Leave it to rest and then see what happens! Based on what you observe, what can say about the density of oil? Then, try stirring the oil and water for 30 seconds then leave to rest, what has happened?


The same applies for gases, if you fill a balloon with helium it will want to rise up through the air. This is because helium has a lower density than air.

When an object is floating on top of water, it is experiencing an upward force that opposes its weight. This force is called the buoyant force. A submarine uses a change in buoyancy to go from floating on the water’s surface to diving down beneath the water. This was the stroke of genius used by John Philip Holland when he invented the submarine.

Let’s take a closer look at how a submarine dives and surfaces