Recommendations on how to do physics homework

Prof. Ernest Barreto

Department of Physics & Astronomy

George Mason University

Doing physics problems develops several very important and useful skills that will help you way beyond your physics education. Most importantly, these include organizing your thinking and communicating your results. These are skills that are acquired only by practice. And lots of it! Below are recommendations for how to do physics homework. Some suggestions will be more relevant than others depending on the specific problem being considered.

Very important: If you are confused about a particular topic, you will probably find that it is difficult to formulate a question to ask. Formulating a good question is 95% of learning physics! The best way to learn is to discuss physics with other people. Talk to your classmates, and do not be afraid to seek help from the professor or the tutor just because you don’t have a specific question. Simply discussing things will help.

It is very common for students to find physics problems to be hard, and then very suddenly, when you make a conceptual breakthrough, they become easy. Strive for this experience and don't give up.

General Recommendations

• Use unbound blank paper. Don’t let the paper you use restrict your thinking.
• Begin by thinking in terms of principles, not in terms of which formulas you need.
• Organize your thinking by drawing a picture of the situation. This is by far the most useful way to approach a problem, and is the “secret” to learning physics.
• Pick a coordinate system and use it. This is how you connect what the problem gives you to the variables in your formulas. You should have a very clear idea of how every term in your formula relates to the coordinate system.
• Make sure you understand what is known and what needs to be determined. Make a list, and assign variable names to the unknowns.
• Make sure you understand the units being used. Convert if needed. All those numbers have a meaning, and sometimes problems pick funny units to try to trick you.
• Believe it or not, it really is easier to manipulate formulas first, then plug in numbers at the end. This makes it easier to go back and correct any mistakes you may make, and is the best way to earn partial credit if you can’t solve the problem completely.
• You need to communicate your line of thinking to the grader. Simply getting the right answer is not enough. Be neat and clear. Don’t be afraid to use words to describe your approach.
• Ask yourself if your answer is reasonable, both in terms of magnitude and units.
• If you need to make assumptions, state them explicitly.
• Staple your pages together. Please.
• Discuss your homework with others. Form study groups.
• There are often several ways to solve a problem. Use the methods described in the chapter to answer problems from that chapter.

Specific Recommendations (these will be useful later on in the semester)

• Keep in mind the difference between scalars and vectors. If your answer is a vector, you must report both a magnitude and a direction.
• Advice on drawing free-body diagrams:
1. It is often helpful to depict the object in question as a point (unless torques are involved). Draw the force vectors as arrows that come out of this point.
2. Only include forces. Other vectors such as acceleration or velocity should not be on a free-body diagram.
3. Only indicate forces that act on the object in question. Forces that the object exerts on other things are not relevant for this diagram.
4. When you draw a force vector, ask yourself: “Where does this force come from? What other object gives rise to it?” If you can’t answer this question, then erase that force! Either that force isn’t real, or you’re not using an inertial reference frame.
5. Don’t blindly refer to “the normal force.” Make sure you can answer the question above before drawing it in.
• Remember that the two forces that Newton’s third law refers to act on two different objects!
• When calculating work, be clear about what force is doing the work, and what object the work is being done on.