While speed, distance and time are the most common compound unit questions you will encounter, You still need to be able to find the others. The one we will focus on here is density, mass and volume.
Density is a measurement of how heavy something is for a given unit of mass. (Effectively, the denser an object is, the heavier it is.
The hardest part of completing questions about Density, Mass and Volume is remembering the formulas.
How to find the formulas.
The easiest way to do this is to memorise the formula triangle. We can then use this to find the formulas.
How do we use this?
Simple, just highlight the thing you are trying to find (I've done it with a red circle) and the formula will be what's left.
Example 1: Find the volume of an object that has a density of 20g/cm3 and a mass of 500 g
The first step in these calculations is always to decide what you are trying to find. For this one, we are directly asked for the volume
V = M ÷ D
Now we can plug in our numbers
V = 500 ÷ 20
The last question that you need to ask is what the units are. The trick here is that they are always the same as one of the other units. The density here is given in g/cm3, so our volume must be cm3.
Distance = 25 cm3.
Example 2: An object has a volume of 75 cm3 and a mass of 15g. Find the average density of the object
Once again, we need to decide which formula. Don't worry that it asks for average density, we can use the same formulas.
D = M ÷ V
Plugging in our numbers gives...
D = 15 ÷ 75 = 0.2
Our units are grams divided by cm3 , or grams divided by cm3 (remember, the units are in the question).
Density = 0.2 g/cm3