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Know Your Variables


Two Big Questions

When you begin a physics problem, it can be intimidating. Frequently there are several components to the problem, and several formulae in the section that you're studying that use these variables. Before you can figure out which formula is most helpful, however, it's important to know two things:
  1. What do you know?
  2. What are you trying to find?

When you start a problem, it can save a lot of headaches if you clearly lay out for yourself - and for the teacher who is grading your homework or test - what variables you are using. Take the information from the problem and define the variables at the top of your workspace. Also read the problem to make sure that you know which variable you are looking for.

Make sure when doing this to keep careful track of the units for each variable and also the significant figures.

Defining Variables: An Example

Consider a very basic, introductory problem in physics: motion in a straight line, under a uniform acceleration.

A motorcycle moving in a straight line under constant acceleration of 4.0 m/s2 passes a street sign. Exactly 5.0 m beyond the signpost, the cyclist's velocity is 15.0 m/s. How far from the signpost is the cyclist when the velocity reaches 25 m/s?

Upon starting the problem, the following might be how I would set up my work:

a = 4.0 m/s2
x0 = 5.0 m
v0 = 15.0 m/s
v = 25.0 m/s
x = ?

Then, to the right of these variables (or beneath them, if you write them across the top of the workspace) you would begin the work. Since you know know for sure which values you're working with, and which value you're looking for, it becomes significantly easier to determine the methods that will work.


This may, perhaps, seem like a somewhat trivial example, but considering this is the sort of problem that would be in the beginning of an introductory physics course, getting a grasp on how these four numbers relate to each other could prove to be challenging to the novice student.

As the problems get more complex, however, it becomes even more important to get yourself organized at the beginning of the problem. There's nothing worse than going through a full page of work only to realize that there was an error in your set-up.

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