Some people wonder why some objects float on water while others sink. The specific gravity of water is quite a complex issue and one of the best ways to understand it is by taking stock of some of the objects that float and those who sink.

Give me a lever long enough and a fulcrum on which to place it, and I shall move the world. – Archimedes

Buoyancy is the main principle we must understand in any approximation of why some objects float while others do not. The first person to understand the importance of buoyancy was the Greek mathematician and inventor Archimedes of Syracuse (c. 287 – 212 BC).

He is credited with having discovered a method for determining the volume of objects that had irregular shapes. This is what we now referred to as Archimedes’ Principle. According to this principle, what buoys up any object is the force that is equal to the weight of the water (or liquid) that has been displaced by it.

**Specific Gravity**

Nothing is softer or more felxible than water, yet nothing can resist it. – Lao Tzu

The name of the force that determines whether an object will sink or float on water is known as specific gravity.

The formula for specific gravity establishes that if the specific gravity of a solid object or a liquid is greater than one, then the object or liquid in question will sink. If, however, the specific gravity is less than one, then the object or liquid in question will float.

**How Is Specific Gravity Calculated?**

The two things that you need to know before you can calculate specific gravity is the density of the solid object or liquid, and the density of the water. In order to calculate their density at Standard Temperature and Pressure (STP), you will need to divide the mass and the volume.

Standard Temperature is 4 degrees Celsius (39 degrees Fahrenheit). Standard pressure is 760.000 mmHg (millimeters of Mercury). In the specific gravity calculation, density is expressed with the Greek symbol ρ pronounced rho. You need to become familiar with all these terms and concepts before you can begin to understand what specific gravity is, let alone how to calculate it. This is actually no different from any other scientific pursuit. Every science has its own specific terminology. The only way to approach science is by first becoming familiarized with all the specialized terminology.

At Standard Temperature and Pressure, the density of water is calculated by dividing the mass of water by the volume of water density of water, which equals 1/1 density of water, which equals 1 gram/cm cubed.

Here is the formula for calculating specific gravity:** SG = (ρ object or liquid)/ ρ water.**

The formula could be rewritten if you already have the specific gravity of a solid object or liquid and what you need to calculate is their density. In which case, the formula would be the following: SG x ρ water = ρ** **object or liquid.

Whichever unit of measurement you use in your calculations is fine just as long as you are consistent and use the same unit of measurement for the density of water and the density of the object or liquid in question. The only thing you have to ensure is that you are consistent in your calculations, by always using the same unit of measurement.

**What Are the Effects of Specific Gravity?**

Specific gravity is the result of comparing the density of an object or liquid to the density of water. But the object or liquid and the water have to be at the exact pressure and the exact temperature.

This is important because any changes in temperature and pressure would affect the density of solid objects and liquids. And any changes in the density of solid objects and liquids will also have an effect on their specific gravity.

So, this means that the specific gravity of any substance (and this includes water) changes depending on both pressure and temperature. You need to control or regulate outside influences in order to avoid any changes in specific gravity.

**Is Specific Gravity the Only Way to Compare the Density of Two Different Substances?**

When it comes to calculating the difference in density between two different substances, there are two scientific approaches. One of them is the specific gravity that we have already discussed. The other one is relative density.

So, how is relative density calculated?

The way to calculate relative density is by dividing the density of both substances. You divide the density of substance number 1 by the density of substance number 2. In this formula, the substance that is usually taken as a reference. The results of that calculation will tell us whether the two substances in question have the same or a different mass. If, for instance, the result of dividing both densities is one, then that would mean that both substances have the equal densities. And it would also mean that if you take equal volumes of those two substances, they both would also have the exact same mass.

Unlike popular belief, science shows us that whether a solid object or liquid will float, or sink is not necessarily related to the material it is made of. It is just down to the density of both the object or liquid and the water that you are using. Anyone with access to the right information about the object in question and the amount of water could calculate their density and, therefore, determine whether the object or liquid will sink or float, just as long as they can also control the temperature and density.

Whether you choose to use specific gravity or relative density, you should be able to use the result of the calculation to determine whether a specific object or liquid will float on water or sink.