# How to Check for Physically Reasonable Answers When Solving Physics Problems

Because physics describes reality, your solutions to any physical problems you address should be able to describe reality as well. You can avoid many errors by checking that your answers have the following characteristics:

**They have the correct units.**If a problem asks you to find a speed and you gain 5 kilograms, then you know that you made a mistake somewhere. (Note that this check only works if you follow your units through the entire problem.)

**They are the right size.**If you calculate that the mass of a planet is 53 grams, that the speed of a soccer ball is 3 trillion meters per second, or that the temperature of the ice is 350 degrees Celsius, start looking for the error.

**They point in the right direction.**When you’re looking for a vector, you sometimes know roughly which direction it should point.

**They have the correct tag.**If you find that the density of a liquid is 1200 kilograms per cubic meter, then you have made a signal error along the way.