Types of friction:

1. Static Friction (f_s):

• Acts when there is no relative motion between surfaces
• Self-adjusting: 0 <= f_s <= mu_s * N
• Direction: opposes TENDENCY of relative motion
• At the verge of motion: f_s = mu_s * N (maximum static friction)

2. Kinetic (Sliding) Friction (f_k):

• Acts when surfaces are in relative motion
• Constant: f_k = mu_k * N
• Direction: opposes relative motion
• mu_k < mu_s (kinetic < static coefficient)

3. Rolling Friction:

• Much smaller than sliding friction
• Usually negligible in JEE unless specified

Angle of Friction (lambda): tan(lambda) = mu_s This is the angle the resultant of N and f_max makes with the normal.

Angle of Repose (alpha): tan(alpha) = mu_s The maximum angle of an incline at which a body remains stationary. Note: angle of repose = angle of friction.

Friction graph: As applied force increases from 0:

• Static friction increases linearly up to f_max = mu_s*N
• At f_max, body begins to move
• Friction drops to f_k = mu_k*N and remains constant