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Friction

Friction acts along the contact surface and opposes sliding. On a horizontal surface the normal force equals the weight, and the friction force is the coefficient of friction times the normal force. The coefficient has no unit. It is larger for rough surfaces than for smooth ones, and it can differ depending on whether the body is at rest or already sliding. Without friction a box on ice would not stop by itself.

R=μNR = \mu Nfriction force
N=mgN = mgnormal force on a horizontal surface

Symbols

RRfriction forceN
μ\mucoefficient of friction
NNnormal forceN

Example

A 20 kg box is pushed on a horizontal floor with coefficient of friction 0.3.

R=0.3⋅20⋅9.81=58.86R = 0.3 \cdot 20 \cdot 9.81 = 58.86 N.

Friction has an upper limit. A push larger than that makes the box slide.
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Part of Physics 1: Forces and Newton's laws.