Free-body diagrams and equilibrium
Isolate the body and replace every contact with the force or moment it applies. For static equilibrium of a planar rigid body, ΣFₓ = 0, ΣFᵧ = 0, and ΣM about any point = 0. Three equations determine at most three unknowns; more supports make the problem statically indeterminate.
Check yourself: Have you included all reaction forces and chosen a convenient moment center?
Friction and impending motion
Static friction satisfies f ≤ μₛN. At impending slip, f = μₛN with direction opposing the tendency to move. Kinetic friction uses f = μₖN once sliding begins. A friction force does not always equal μN; it adjusts to maintain equilibrium until the limit is reached.
Check yourself: Is the body actually on the verge of slipping, or is friction below its limit?
Kinematics of particles
Position, velocity, and acceleration are related by differentiation: v = ds/dt, a = dv/dt. For constant acceleration, v = v₀ + at and s = s₀ + v₀t + ½at². For curvilinear motion, resolve into tangential (speed change) and normal (direction change) components: aₜ = dv/dt, aₙ = v²/r.
Check yourself: Have you used a consistent sign convention for displacement and acceleration?
Newton's laws for particles and systems
ΣF = ma applies to each particle or to the mass center of a system. For a system of particles, internal forces cancel in pairs. Weight acts at the center of gravity. For rigid-body plane motion, additionally ΣM_G = I_G α about the mass center or ΣM_O = I_O α about a fixed point O.
Check yourself: Is the reference point for moments either the mass center or a fixed point?
Work-energy and impulse-momentum
The work-energy theorem states: net work = ΔKE. Potential energy from gravity is mgh. Impulse equals change in momentum: ∫F dt = Δ(mv). For collisions, the coefficient of restitution e relates separation speed to approach speed along the line of impact. Momentum is conserved when external impulse is negligible.
Check yourself: Have you included work done by all forces, including friction and gravity?