Static Friction and Limiting Equilibrium
Static friction obeys |f_s| <= mu_s N and opposes the impending relative motion.
Why this shows up in the exam
Impending motion · Wall contact · Toppling checks
Learn the idea
Static friction adjusts up to mu_s N and reaches it only at impending slip. A resting contact uses only the friction needed to prevent relative motion.
🧠 Memory hook: Static friction supplies what is needed, up to its limit.
Get this one clearly and it pays off every single time it shows up in the paper. 🎯
Formulas & facts to keep ready
- |f_s| <= mu_s N — static range
- |f_s| = mu_s N — limiting equilibrium
- tan(theta_r) = mu_s — simple angle of repose
How to approach it
- 1Assume slip tendency
- 2Find required friction
- 3Check limit and other failure modes
Common slip-ups that cost marks
- •Always using mu_s N
- •Wrong impending direction
- •Ignoring toppling
🌟 That's the whole idea — you've got this. Try the practice set below; every question you attempt makes it stick a little harder.
Original chapter practice
Original questions for this chapter, not past-paper questions or an exact mapping to this individual concept.
A 2 kg block moves on a horizontal rough surface with coefficient of kinetic friction 0.2. A horizontal force of 10 N acts on it. Take g = 10 m/s^2. What is its acceleration?
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