Key Concepts: Friction & Tension

Friction

• Static friction (fₛ): Prevents an object from starting to move. fₛ ≤ μₛN (up to a maximum).
• Kinetic friction (fₖ): Acts on a sliding object. fₖ = μₖN (constant).
• μₛ > μₖ — it takes more force to start motion than to maintain it.
• N = normal force; μ = coefficient of friction (dimensionless, depends on surfaces).

Tension

• Tension is the pulling force transmitted through a string, rope, or cable.
• In an ideal (massless, inextensible) string, tension is the same throughout.
• A pulley changes the direction of tension but not its magnitude (ideal pulley).

Solving Problems

• Draw a free-body diagram for each object.
• Apply ΣF = ma to each object separately.
• For connected objects, they share the same acceleration (if the string is taut).