This page gives you the key ideas to recognise before you start a question. The full course then takes you through the method, exam-style practice and fully worked solutions.
Isolate the body
Decide which object or system you are analysing. Only draw forces acting on that chosen body.
Label forces
Common forces include weight, normal reaction, tension, thrust and resistance. Show their directions clearly.
Resolve if needed
Choose useful directions and resolve forces consistently. On a straight horizontal or vertical problem this may be immediate.
Apply F = ma
Use the resultant force in a chosen direction, not an individual force unless it is the only force in that direction.
Worked example: horizontal motion
A 2 kg particle has a resultant horizontal force of 10 N.
F = ma
10 = 2a
a = 5 m s⁻²
F = ma
10 = 2a
a = 5 m s⁻²
Common mistakes to avoid
- Writing F = ma using one force rather than the resultant force.
- Putting acceleration on a force diagram as though it were a force.
- Forgetting that weight is mg, not simply the mass.