Friction Types — Comparison Note — Notes

Feature	Static Friction ( $f_s$ )	Kinetic Friction ( $f_k$ )	Rolling Friction ( $f_r$ )
Formula	0 ≤ $f_s$ ≤ μ_s N	$f_k$ = μ_k N	$f_r$ = μ_r N
Self-adjusting?	YES — matches applied force	NO — fixed at μ_k N	NO — fixed at μ_r N
Coefficient range	μ_s: 0.1–1.0 typically	μ_k < μ_s always	μ_r << μ_k
When it acts	Before relative motion starts	During sliding motion	During rolling without slipping
Depends on speed?	No	No (approximately)	No
Direction	Opposes tendency of motion	Opposes direction of sliding	Opposes rolling direction
NEET trap	Assuming $f_s$ = μ_s N always	Forgetting μ_k < μ_s	Rarely tested; usually negligible

Key Insight: Since μ_s > μ_k, it is harder to START an object moving than to KEEP it moving once sliding. Rolling friction is so small that μ_r is effectively negligible in most NEET problems unless explicitly given.