An object at rest will stay at rest, and an object moving with constant velocitywill continue to move with constant velocity, unless an unbalanced force acts on the object.
What is the resultant force and an unbalanced force?
The resultantforce is the sumofforces acting on a particle – consider it as a singleforce that achieves the sameresult as all the forces combined
An unbalancedforce is a force acting on a particle that is not cancelled by another force acting in the opposite direction
So a non-zero resultant force will be unbalanced (hence the wording in Newton’s First Law of Motion) and the particle will accelerate
What does equilibrium mean?
A particle is in equilibrium if the resultantforce acting on it is zero
In other words when the sumoftheforces acting on a particle is zero
For example, in the horizontal direction, any forces acting to move the particle to the left will be balanced by any forces acting to move the particle to the right.
There does not need to be the same number of forces in both directions – two forces could be acting to move the particle to the left but are cancelled out by only one force acting to move the particle to the right.
It is unlikely you will get an exam question that only deals with one dimension at AS and A level. However two-dimensional problems can often be broken down into two one-dimensional problems so the principles in this note are important to understand.
Paul has taught mathematics for 20 years and has been an examiner for Edexcel for over a decade. GCSE, A level, pure, mechanics, statistics, discrete – if it’s in a Maths exam, Paul will know about it. Paul is a passionate fan of clear and colourful notes with fascinating diagrams – one of the many reasons he is excited to be a member of the SME team.