Modelling Assumptions
Mechanics has a standard vocabulary of simplifying assumptions — “particle”, “smooth”, “light”, “rigid” and so on. Each word removes a real-world complication, and you must know exactly what each one lets you ignore.
What you'll be able to do
- Know the common modelling assumptions
- State what each assumption removes
- Choose appropriate assumptions for a situation
- Recognise the effect of each assumption
The standard assumptions
Each technical word is shorthand for an assumption: a has no size (ignore dimensions/rotation); means no mass (ignore the object’s weight); means no friction; means a string does not stretch; means a body does not bend.
What each removes
These assumptions simplify the forces involved: “smooth” removes the friction force; “light” means tension is the same throughout a string and you ignore its weight; “particle” lets you treat all forces as acting at a single point.
Tip — When you read “smooth”, immediately note: no friction force in your diagram.
Choosing assumptions
You pick assumptions that make the problem solvable while staying realistic enough. The exam often asks how a result would change if an assumption were dropped (e.g. including friction would reduce acceleration).
Formula recap
Common mistakes to avoid
Key takeaways
- Particle = no size; light = no mass; smooth = no friction.
- Inextensible = does not stretch; rigid = does not bend.
- Each assumption removes a force or complication to simplify the model.
Test yourself
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