What is Normal force and friction? — Physics

Definition 25.11 High School Physics · Chapter 25 — Newton’s Laws

A surface acts on a body through two components: the normal force $\vect N$ , perpendicular to the surface, and friction $\vect f$ , tangential. Experiment gives the friction laws: while the body does not slide, static friction adjusts itself up to a ceiling, $f_s \leq \mu_s N$ ; once it slides, kinetic friction is $f_k = \mu_k N$ , against the sliding. The coefficients of friction $\mu_s$ and $\mu_k$ ( $\mu_k \leq \mu_s$ ) depend only on the two materials.

Free-body diagram on the incline: weight P, normal force N, friction f up the slope for a block sliding down. Axes along and perpendicular to the slope split P into mg and mg (dashed). — Free-body diagram on the incline: weight $\vect P$ , normal force $\vect N$ , friction $\vect f$ up the slope for a block sliding down. Axes along and perpendicular to the slope split $\vect P$ into $mg\sin\alpha$ and $mg\cos\alpha$ (dashed).

Examples

Example 25.12 (The incline, twice)

A block slides down a slope of angle $\alpha$ ; axes: $x$ down the slope, $y$ perpendicular. On $y$ : $N = mg\cos\alpha$ . On $x$ , frictionless: $ma = mg\sin\alpha$ , so $a = g\sin\alpha$ — no mass; at $\alpha = 30^\circ$ , $a = 4.9\,\mathrm{m}/\mathrm{s}^{2}$ . With kinetic friction: $a = g(\sin\alpha - \mu_k\cos\alpha)$ ; for $\mu_k = 0.20$ , $a = 9.81\,(0.500 - 0.173) = 3.2\,\mathrm{m}/\mathrm{s}^{2}$ . And it stayed put in the first place only while the needed static friction fit under its ceiling: $\tan\alpha \leq \mu_s$ .

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