S2.3 How to use Newton`s Laws
This section gathers together some hints you will find useful when applying Newton’s Laws. They complement the general problem-solving guidelines set out in S0.5. They are best learned by working through the examples which follow.- Choose your system: Newton’s 2nd Law can be applied to any ‘bit’ of the universe. You must choose the ‘bit’: this is your system. It may be all or only part of the physical system described in the problem. It is a good idea to draw a box around it to remind you what you have chosen.
Identify all the forces: exerted on the chosen system by anything else. Other forces are irrelevant to the behaviour of the system you have chosen.
- Draw a vector diagram: showing all the forces you have identified; each force should be represented by a vector with its origin on the system. This is called a free-body diagram.
Apply the 2nd Law to your chosen system: to establish the relationship between the net force it experiences and its acceleration. Then:
- If the acceleration is given, the task is to infer something about the forces.
- If the forces are given the task is to find and solve the equation for the acceleration. This is the equation of motion. Its solution describes the way the position coordinates of the system change with time.
- Remember that forces are vectors: …and have to be combined accordingly. In doing so you can make life easier for yourself by choosing your coordinate axes wisely.
- Remember that the 3rd law is different: in that it always involves two systems. One of the forces in the third law pair acts on one system; the other force acts on the other system.
Learning Resources
![]() | HRW sets out its own set of Problem Solving Tactics, in relation to Newton’s Laws, in Chapter 5.9 |
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