Forces · Topic 6 of 17

Friction

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Physics · forces

Friction

Friction opposes sliding. Static friction holds a still object, kinetic friction slows a sliding one; both follow f = μN, and static is usually larger.

01 · What it isFriction opposes motion

02 · The formulaf = μN

03 · Static vs kineticμₛ ≥ μₖ: starting is harder

What it is

Friction opposes sliding, along the surface, against the motion must know

μ has no units, usually 0 to 1, and depends on the two surfaces

Friction also gives grip — walking, tires, rubber soles

Know by heart

Friction f = μN (kinetic μₖN; static up to μₛN) must know

The rules

μₛ ≥ μₖ (usually >): starting a slide takes more force than sliding must know

Static friction matches the push up to μₛN, then the object slips

Friction depends on materials and normal force, not contact area

Bigger normal force (heavier, pressed harder) → more friction

Lubricant, water or ice → lower μ, less grip

Sliding at constant velocity → push = kinetic friction

Worked examples

25 kg, μₖ = 0.2: N = 245 N → friction = 49 N

40 kg, μₛ = 0.5: must push more than 0.5 × 392 = 196 N to start it

12 kg, 90 N push, μₖ = 0.5: friction 58.8 N, net 31.2 N → 2.6 m/s²

Don't confuse

Static = not sliding yet; kinetic = already sliding must know

Static or kinetic friction?

Commonly confused

	Static	Kinetic
When	Surfaces not sliding	Surfaces sliding
Size	Matches the push, up to μₛN	Fixed at μₖN
Coefficient	μₛ, usually larger	μₖ, usually smaller
Picture	A crate that won't budge	A crate sliding across the floor

Also know

Net force = applied push − friction, then a = F_net ÷ m

More detail

Show 3 more facts

Kinetic friction barely changes with speed

Synovial joints: μ ≈ 0.015, lower than shoes on ice

Friction turns motion energy into heat (rubbed surfaces warm)

Pearl

Getting a heavy box moving takes more push than keeping it moving: static friction beats kinetic.

Sources · OpenStax College Physics 2e §5.1 Friction · OpenStax College Physics 2e §4.5 Normal, Tension, and Other Examples of Forces · Union Test Prep — HESI Physics study guide, p. 1: Friction · NurseHub — HESI A2 Physics course (topic list: Friction) · Built from the verified NurseSavvy HESI A2 fact map (facts reviewed 2026-10-16). HESI is a trademark of Elsevier; NurseSavvy is not affiliated with Elsevier.

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