Unit Analysis
In physics, it is not uncommon to change from one system of measurement to another. The best method to
accomplish this change is unit analysis.
In physics calculations, the units are very important and must be treated in the same way as constants and
variables are treated in algebra.
Notice in the example how the units multiply, divide and cross cancel just like numbers and variables.
Conversions
Physics calculations require close examination of the units. Therefore, you must know the conversion factors
that allow you to make the desired calculations.
Common Conversion Factors
1 inch = 2.54 centimeters 
1 foot = 12 inches 
1 pound = 454 grams 
1 liter = 1.057 quarts 
1 yard = 3 feet 
1 hour = 60 minutes 
1 calorie = 4.18 joules 
1 mile = 5,280 feet 
1 minute = 60 seconds 
1 atm = 101.3 kilopascals 
1 mile = 1,760 yards 
1 pound = 16 ounces 
This Per That Problems
The common type of physics problem is a "this per that" or what is called a conversion
problem. An example of a this per that would be 12 inches per 1 foot.
The hardest part of a this per that is knowing the conversion factor (this
per that). Once you know the conversion factor all you have to do is set up the equation, verify your units, and
plug&chug (put it in the calculator).
Example
There are 500 monkeys and you can put 5 monkeys in a cage.
How many cages will you need?
500 monkeys 5 monkeys per cage How many cage?
Reasoning tells me that if I divide 500 by 5 I will need 100 cages.
The above example is a this per that problem and similar in logic to the
types of problems you will do in physics. So, let's look at it set up like a physics problem.
Notice that monkeys cancel out and cages is our new unit.
