What is assumed about a particle?
The object has no size and rotation is ignored.
What is assumed about a light string or rod?
Its mass is negligible (zero).
What is assumed about an inextensible string?
The string does not stretch
What is assumed about a smooth surface?
No friction acts between the object and the surface.
What is assumed about a rough surface?
Friction acts and must be modelled (usually using ( F \leq \mu R )).
What is assumed about a smooth pulley?
The pulley has no friction and changes the direction of the tension only.
What is assumed about a light pulley?
The pulley’s mass is negligible and does not affect the motion.
What is assumed about gravity?
Gravity is constant and acts vertically downward with ( g = 9.8 \
What is assumed when a body is in equilibrium?
The resultant force (and often moment) is zero.
What is assumed about air resistance if not mentioned?
It is neglected (ignored).
What does it mean to model an object as a rigid body?
It does not bend or deform; distances within it remain fixed.
What does it mean to model motion in one dimension?
Only horizontal or vertical motion is considered — not both at once.
What does it mean to use a uniform rod?
The mass is evenly distributed along its length.
What does ‘smooth contact’ mean between two objects?
No friction between them.
What does it mean if a string goes slack?
Tension becomes zero; it no longer affects motion.
What does it mean to treat acceleration as constant?
The net force remains unchanged throughout the motion.
Why is modelling important in mechanics?
It allows real-world problems to be simplified into solvable mathematical models.
What limits the validity of a mathematical model?
Breakdown of assumptions (e.g.
What is the effect of modelling an object as a point mass?
All mass is concentrated at a single point — simplifies calculations.
Why might a model fail for large values of time or distance?
Because real-world effects like friction
