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Friction

Friction is a reaction force along the contact surface that resists sliding. Static friction is not fixed but exactly as large as equilibrium requires, up to a limit $\mu_s N$ . Once the body slides, friction is kinetic and usually a bit lower.

F \le \mu_s N

static friction (before sliding)

F = \mu_k N

kinetic friction (while sliding)

Symbols

$F$	friction force	N
$N$	normal force	N
$\mu_s,\ \mu_k$	static/kinetic friction coefficient

Example

A 10 kg block, $\mu_s = 0.4$ , resting on a flat surface:

$F_{max} = \mu_s N = 0.4\cdot 10\cdot 9.81 \approx 39.2$ N.

Always find

N

from equilibrium perpendicular to the surface – it is not always equal to the weight.

Practise trusses and friction for free →

← Method of sections · Centroid →

Part of Statics and Dynamics: Trusses and friction.