How to start any forces / mechanics question.

2 min read

31 March 2023

When you see a mechanics question in physics, especially those involving forces – it’s often hard to know where to start.

There are of course the easy ones, especially at GCSE. The ones where you get given THE force, THE mass and have to work out the acceleration. You may even be given the formula (F=ma in case you don’t remember your favourite formula…which it is! You have no choice!!!)

My tutoring style is based on giving you a great grasp of the fundamentals. The tools in your toolkit so to speak. Such a deep understanding, knowledge and familiarity that once you know what the problem is and the strategy you will use to answer it, it’s easy. Sometimes though, knowing whether to use forces, momentum/impluse, energy, circular motion (the last one is A-level only you GCSE people, so don’t worry).

Here’s what I do, and what questions I ask to help figure out my first approach, my first method/strategy.

  1. Is it in equilibrium? That means is it accelerating. The speed can be constant, but if the direction is changing then it’s accelerating. If it is accelerating, it’s not in equilibrium. If the velocity is changing, if there is an overall resultant force, it’s not in equilibrium. Note it CAN be moving and be in equilibrium – but must be travelling at a constant speed, and in the same direction.
  2. If it’s not, it’s accelerating in the direction of the resultant force. So the velocity is being added to in that direction each second.
    Also the magnitude of this acceleration is the resultant force in Newtons divided by the mass of the object in kg. F=ma – your favourite remember?
  3. If it IS in equilibrium, then the acceleration is 0, the velocity is constant and use the equation s=vt or distance=speed*time
  4. If it has a constant resultant force, i.e. a constant acceleation (F=ma means they are linked) – then use the SUVAT equations.
  5. A-level only here: If it isn’t in equilibrium, and the acceleration/resultant force isn’t constant – it’s v=ds/dt. a=dv/dt or the corresponding two integrals.

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