In one line
Friction is the resisting force along the contact surface that opposes relative motion, and its limiting value equals the coefficient of friction times the normal reaction.
Explained simply
Push a heavy cupboard: at first nothing moves because friction matches your push. Push harder and at some point it suddenly starts to slide; that maximum is limiting friction. Once it moves, it feels slightly easier to keep it moving: that is kinetic friction.
Technically, static friction is self-adjusting from zero up to the limiting value \( F_{max} = \mu_s N \); during sliding, kinetic friction \( F_k = \mu_k N \) with \( \mu_k \) slightly less than \( \mu_s \).
How it works
- Laws of static friction: (a) friction always acts opposite to the direction in which the body tends to move; (b) it equals the applied force until limiting friction is reached; (c) limiting friction is proportional to the normal reaction, \( F = \mu N \); (d) it does not depend on the area of contact; (e) it depends on the nature and condition of the surfaces.
- Laws of kinetic friction: (a) acts opposite to the motion; (b) \( F_k = \mu_k N \), slightly less than limiting friction; (c) roughly independent of speed for moderate speeds; (d) independent of contact area.
- When the problem says 'about to move' or 'impending motion', use \( F = \mu N \). Otherwise friction is only what equilibrium needs.
Key formula
\( \mu = \dfrac{F}{N} \), dimensionless. F = limiting friction (N), N = normal reaction (N).
Quick example
A 400 N block on a floor with \( \mu_s = 0.3 \): limiting friction \( = 0.3 \times 400 = 120 \) N. A horizontal push of 80 N will not move it, and friction then is only 80 N, not 120 N.
Common mistakes
- Writing \( F = \mu N \) when the body is not at the point of slipping.
- Assuming N always equals the weight; N changes on inclines or when the pull is inclined.
- Showing friction in the direction of motion.
How MU asks it
A standard 3 to 4-mark theory part: 'State the laws of friction' or 'Differentiate between static and kinetic friction'. List the laws in numbered points and give \( F = \mu N \). The laws then underpin all friction numericals.