Common Forces in Mechanics

subtopiceasy~15 min study

Details the characteristics and representation of forces such as normal reaction, tension, and weight, crucial for FBDs.

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What is Common Forces in Mechanics?

Contact force perpendicular to a surface; N = mg on a horizontal floor and N = mg cos(θ) on an incline

Key formula / rule: Friction

Key points

Identify weight, normal reaction, tension, spring force and friction, and apply the limiting friction condition

Common exam trap

Using f = μ N before the body is on the verge of sliding

Definitions

Name: Normal reaction
Definition: Contact force perpendicular to a surface; N = mg on a horizontal floor and N = mg cos(θ) on an incline

Name: Friction
Definition: Tangential contact force opposing relative sliding; static friction is self-adjusting up to f_s(max) = μ_s N, kinetic friction is f_k = μ_k N

Name: Exam insight
Definition: μ_k is less than μ_s, and friction acts up the incline when a body tends to slide down

Name: Worked example
Definition: 5 kg block, μ_s = 0.4: motion begins only beyond f_s(max) = 0.4 x 5 x 9.8 = 19.6 N

Learning objectives

Identify weight, normal reaction, tension, spring force and friction, and apply the limiting friction condition

Formulae

Name: Friction
Expression: f_s(max) = μ_s N; f_k = μ_k N

Name: Spring force (Hooke's law)
Expression: F = -k x

Common mistakes

Using f = μ N before the body is on the verge of sliding
Assuming tension differs across a light frictionless pulley

Keywords

normal reaction
friction
tension
spring force
weight
coefficient of friction
contact force