How to Apply the Correct Physics Formula to an Equation

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How do you know what is the right formula to use for an equation? Follow these steps to make sure you’re applying the correct formula.

Identifying the Correct Formula

The first step in this regard is to ‘correctly’ identify the formula:

1. Scan the question to see what physical quantity is to be calculated. Is it length, force, etc.?

2. Identify the specifics of the unknown. For example, is it the length between A & B? Is it the force by P?

3. Now think of a formula that relates this quantity with the quantities known in the question. It could be that the quantity you are required to calculate is not given by any single formula directly. In that case, you may have to apply more than one formula to reach the result.

4. This one’s a crucial step. Try to recall the precise condition for which the formula has been designed. Here are a few things that might be specified:

  • Be careful about the symbols used in an equation. Sometimes a symbol used in the equation may refer to a physical quantity that is different from what the symbol usually represents.
  • Often, the lowercase and uppercase of the same letter or symbol may represent different physical quantities. Be sure to correctly recollect the name, representation and the case of the symbols used in the equation.
  • Values of the constants used in the equation may be universal or may be specified for some particular conditions. The values of most universal constants are given in the question or paper itself.
  • A formula is generally meant for a particular condition imposed on some of the variables that may or may not be present in the equation. In fact, the question may state certain values or conditions only to justify the formula to be used.
  • Some assumptions may have been taken during the derivation of the formula which, if violated in the condition described in the question, makes the formula inapplicable.

Putting in the Values

Now move on to the second task: correctly substituting the information given in the question into the formula.

  1. Read the values correctly. Double check the numbers when you copy them. It’s a bit discouraging when you take the trouble of solving the entire equation and discover in the end that you substituted incorrectly.
  2. Use the proper relations to obtain the value of the quantity mentioned in the equation from the value of the quantity given in the question.
  3. Don’t forget your unit conversions! Bring all your quantities to a common system of units like the SI system or f-p-s system. This way you automatically know the units of your answer.

Having done the above, you only need to calculate the answer. The formula application part is over. I can’t emphasize enough the need of being careful at every step. There are innumerable ways to make mistakes while using formulas and being cautious is the biggest weapon you have.

Remembering the Formulas

Here are some final tips:

  1. Formula Sheets: It is important to list down all your formulas in one place. You can use a website that lists the formulas or even better, write them down yourself.
  2. Form a ‘clear picture’ in your mind of the condition under which the formula is to be applied.
  3. Write and practice the formulas. Pay attention to the case of the symbols. Along with the formula, always write down all the assumptions and the key to all the symbols used.
  4. Whenever you read a formula, try to connect each of the symbols with the quantities they represent.

So there you go! It’s that simple and time you cracked all the problems in your physics class.