One of the most common questions that you are likely to get when looking at compound units are speed, distance and time calculation (the others being density, mass, volume and pressure,force, area).
The hardest part of completing questions about speed, distance and time 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: A car travels at 40 mph for 3 hours. How far does it go?
The first step in these calculations is always to decide what you are trying to find. For this one, we are asked 'how far...' so this is a distance question.
D = S x T
Now we can plug in our numbers
D = 40 x 3 = 120
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 speed here is given in miles per hour, so the distance must be in miles.
Distance = 120 miles.
Example 2: A bike travels 100 m in 5 seconds. What is it's average speed?
Once again, we need to decide which formula. Don't worry that it asks for average speed, we can use the same formulas.
S = D ÷ T
Plugging in our numbers gives...
S = 100 ÷ 5 = 20
Our units are metres divided by seconds, or metres per second (remember, the units are in the question).
Speed = 20 m/s