How to read a physics problem

Units, a diagram and a sanity check catch most mistakes before they cost points. The five-step routine for every problem.

Physics problems are stories with numbers hidden in them. The routine below turns the story into equations reliably, and it is what graders look for when they give partial credit.

1. Draw it. Even a stick figure and an arrow. For forces, a free-body diagram is not optional; it is the solution.
2. List the givens with units, and the unknown. Convert everything to SI now (km/h to m/s, grams to kilograms). Half of wrong answers are unit mistakes.
3. Choose the principle. Constant acceleration? Kinematics. Forces? Newton's second law. No time in the question? Energy. Collision? Momentum. Say it out loud.
4. Solve symbolically first, then plug in numbers. Algebra with letters is easier to check and gets full credit even if the arithmetic slips.
5. Sanity check. Are the units right? Is the sign right? Is the size plausible? A car going 300 m/s or a mass that came out negative is a signal to look again.

Common traps

• Mixing up "speed" and "velocity" in signs: pick a positive direction and write it on the diagram.
• Using g as 9.8 in one place and 10 in another.
• Forgetting that at the top of a throw the velocity is zero but the acceleration is not.
• Applying energy conservation when friction is doing work.