Friction

This lesson covers the mechanics of friction at A-Level. Friction is a contact force that opposes the tendency of surfaces to slide relative to one another. Understanding the friction model is essential for solving problems involving objects on rough surfaces.

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The Friction Model

The frictional force $F$F between two surfaces obeys the inequality:

$F \leq \mu R$F≤μR

where:

• $F$F is the frictional force
• $\mu$μ is the coefficient of friction (a constant that depends on the surfaces in contact)
• $R$R is the normal reaction force between the surfaces

Two Regimes

Situation	Friction	Condition
Static (not moving or about to move)	$F \leq \mu R$F≤μR	Friction takes whatever value is needed to prevent motion, up to the limiting value
Limiting equilibrium (about to move)	$F = \mu R$F=μR	Friction is at its maximum value
Dynamic (sliding)	$F = \mu R$F=μR	Friction equals the limiting value (A-Level model assumes static and dynamic coefficients are equal)