JEE/NEET Physics · Laws of Motion series · Part 4 of 8 · All parts →
- Friction opposes RELATIVE SLIDING (or its attempt) along a surface
- Static friction: adjusts from 0 up to f_max = μ_s N — only as much as needed
- Kinetic friction: fixed value μ_k N once sliding, slightly smaller than static
- μ_k < μ_s — harder to start than to keep going
- On horizontal ground f = μN; on inclines N = mg cosθ
Without friction you couldn’t walk, drive, write, or sit on a chair — you’d be a permanently sliding object. Friction is the quiet hero of daily life and the most tested ‘annoying force’ in JEE/NEET. Part 4 of the Laws of Motion series.
- What friction really opposes
- Static vs kinetic
- The coefficient of laziness
- Friction on inclines
- Solved examples
- Common mistakes
- This physics in your daily life
- Practice set
- Recap
What Friction Really Opposes
Friction’s job description: oppose relative sliding between two surfaces — actual sliding, or the intention of it. Push a heavy cabinet gently and it doesn’t move: static friction matched you exactly. Push harder; it matches harder — until its limit, and then the cabinet lurches (and moving, friction drops slightly).
Static vs Kinetic
| Static (no sliding yet) | Kinetic (sliding) | |
|---|---|---|
| Size | whatever is needed, up to μ_s N | fixed at μ_k N |
| Direction | opposes intended slide | opposes actual slide |
| Typical μ | 0.6 (rubber-dry road) | 0.4 (rubber-dry road) |
The Coefficient of Laziness
| Letter | What it means (plain words) | Value / unit |
|---|---|---|
| f | friction force (along the surface) | N |
| μ_s, μ_k | static / kinetic friction coefficients | unitless, μ_k < μ_s |
| N | normal force pressing surfaces together | N — NOT always mg! |
Friction on Inclines
On a slope, N = mg cos θ (only the into-surface part presses), so friction’s budget is μ mg cos θ — shrinking as the slope steepens, while the pull mg sin θ grows. At the angle of repose tan⁻¹μ_s they balance exactly: the angle where objects just begin to slide.
Solved Examples
f = μmg = 5000 N → a = 5000/1000 = 5 m/s².
✔
Answer: 5 m/s²
Available friction: 0.4 × 50 = 20 N < 25 N push → it slides.
Net = 25 − μ_k(50); with μ_k = 0.3: net 10 N, a = 2 m/s². ✔
Answer: Slides; net force = push − μ_k N
tan θ = μ_s at the slide point → μ_s = tan45° = 1.
Reverse use: measure the slide angle, read off the texture. ✔
Answer: μ_s = 1
- Always writing f = μN. Static friction is ≤ μ_s N — a ceiling, not a fixed value. It equals your push until the limit.
- Using mg for N everywhere. On inclines or lifts, N = mg cosθ or m(g±a): friction follows the real N.
- Friction ‘always slows things’. It opposes RELATIVE sliding — friction is what pushes your feet (and car tyres) FORWARD when you accelerate.
- Missing that rolling < sliding. Wheels roll precisely to avoid kinetic sliding friction: the ball-bearing principle in one line.
This Physics in Your Daily Life
- Walking is friction driving you forward — push the ground backwards, friction pushes you ahead. Ice (μ ≈ 0.05) removes the bargain and walking becomes comedy.
- Anti-lock brakes (ABS) — keep tyres ROLLING to stay in the static-friction regime (μ_s > μ_k): steering survives braking because static friction can point sideways too.
- Matchsticks and violin bows — ignition and music both live on the static→kinetic friction drop: stick-slip motion made audible or flammable.
- Tyre tread patterns — water-channeling grooves keep μ high in rain: tread depth is literally a friction budget written in rubber.
- Nails, screws and knots — hold forever on static friction alone: your entire house is friction-assembled, no glue required.
Zoom to the microscopic: two ‘flat’ surfaces are mountain ranges in contact at only scattered peaks. Sliding means shearing and climbing these nano-peaks — the resistance of that ugly terrain is friction. Press harder, more peaks touch, more resistance: why f ∝ N. The ‘coefficient’ is the mountain range’s roughness rating.
Push a 50 N box: at 10 N, friction replies 10; at 20, replies 20; at 25 (μ_s = 0.5 × 50)… it replies 25; at 26, the ceiling cracks — box moves, friction drops to 20 (μ_k = 0.4). The last newton before motion is the hardest: the famous jolt when something finally gives.
Graph friction against your push: a straight line at 45° climbing with the push until the ceiling μ_s N, then a cliff-drop to the flat line μ_k N. The graph’s shape IS the physics: adjustable, capped, then slightly cheaper once moving.
Practice set (answers hidden — try first)
(NEET-level) 20 N push on a box needing μ_s N = 25 N to slide:
(JEE Main-level) 2 kg block, μ_k = 0.25 (g=10): kinetic friction =
(NEET-level) μ_s = 0.5: angle of repose ≈
(Concept) Friction acting on accelerating car tyres points:
(JEE Main-level) Skid deceleration with μ = 0.6 (g=10):
- friction opposes relative sliding
- static: 0 up to μ_s N (as needed); kinetic: μ_k N fixed
- μ_k < μ_s: harder to start
- N = mg cosθ on inclines
- angle of repose: tanθ = μ_s
- 🔁 static vs kinetic behaviour
- 🔁 f ≤ μ_s N; f = μ_k N
- 🔁 inclines: N = mg cosθ
- 🧠 Chant: ‘static adjusts, kinetic is fixed’.
- 🧠 Repose: ‘tan of the slide angle = μ’.
- 🏠 Daily: walking = friction pushing you forward.
- 🏠 Daily: ABS keeps tyres in the static regime.
Quick revision
- Friction opposes RELATIVE SLIDING (or its attempt) along a surface
- Static friction: adjusts from 0 up to f_max = μ_s N — only as much as needed
- Kinetic friction: fixed value μ_k N once sliding, slightly smaller than static
- μ_k < μ_s — harder to start than to keep going
- On horizontal ground f = μN; on inclines N = mg cosθ
- What friction really opposes
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