All courses › Foundations of Physics › Friction

Friction

Friction is a force along the contact surface that opposes a body sliding or starting to slide. As long as the body is at rest, the friction adjusts itself and can never exceed $\mu_sN$ . Once it slides, the friction is constant at $\mu_kN$ , directed opposite to the sliding motion.

R \le \mu_sN

static friction (body at rest)

R = \mu_kN

kinetic friction (body sliding)

Symbols

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

Example

A 10 kg block slides with $\mu_k = 0.25$ . The normal force is $N = 98.1$ N, so the friction is $R = 0.25\cdot 98.1 \approx 24.5$ N.

Use

\mu_s

to check whether something starts to slide, and

\mu_k

(usually smaller) while it is sliding.

Practise forces and Newton's laws for free →

← Weight and normal force · Inclined plane →

Part of Foundations of Physics: Forces and Newton's laws.