Why F = ma is the only equation you need to memorise

Every mechanics question on Paper 2 and Paper 4 reduces to Newton’s second law plus a diagram. Students lose marks not because they forgot an equation, but because they skipped the free-body diagram that tells them which equation to write.

The three-step routine

1. Draw the body. Draw every force. Label each one.
2. Choose axes so that as many forces as possible lie along them.
3. Write ΣF = ma along each axis — and only then substitute numbers.

Do that for 40 questions and it becomes automatic. That is what we mean by first principles.