Kinetic and Static Friction
Introduced| English |
|---|
| static friction/ˈstætɪk ˈfrɪkʃn/ |
| kinetic friction/kɪˈnetɪk ˈfrɪkʃn/ |
| coefficient of static friction/ˌkəʊɪˈfɪʃənt ɒv ˈstætɪk ˈfrɪkʃn/ |
It won't budge... then suddenly it slides
- Push a heavy crate gently and nothing happens; push harder and still nothing; push harder still — it lurches into motion.
- Before it moves, static friction 静摩擦 grows to match your push exactly.
- Once it slides, kinetic friction 动摩擦 takes over — usually a little weaker.
- Both come from surfaces gripping each other, and both act parallel to the surface.
Static friction adjusts itself
- Static friction opposes an object that is not yet moving.
- It grows to match the applied push, up to a maximum: $f_s \le \mu_s N$.
- Below that maximum the object stays put; the friction is only as big as it needs to be.
- $\mu_s$ is the coefficient of static friction 静摩擦系数, and $N$ is the normal force.
Which friction acts on an object that is being pushed but has not started to move?
Before sliding, static friction acts and grows to match the push, up to $\mu_s N$.
Kinetic friction is roughly steady
- Once sliding starts, kinetic friction acts: $f_k = \mu_k N$.
- It stays roughly constant no matter how fast the object slides.
- Usually $\mu_k < \mu_s$ — it takes more force to start sliding than to keep sliding.
- That is why the crate "breaks free" and then feels easier to push.
Friction on a slope
Raise the angle or the friction coefficient and see when the block finally starts to slide.
A sliding box has a normal force $N = 40\ \text{N}$ and $\mu_k = 0.3$. What is the kinetic friction force, in $\text{N}$?
$f_k = \mu_k N = 0.3 \times 40 = 12\ \text{N}$.
For most surfaces, how do the coefficients compare?
Usually $\mu_s > \mu_k$, so it takes more force to break free than to keep sliding.
What friction depends on
- Friction depends on the normal force $N$ and the surfaces, through $f = \mu N$.
- Press harder (larger $N$) and friction grows; that is why heavy things are hard to slide.
- Surprisingly, it barely depends on the contact area — a brick slides the same on any face.
- It does not depend on speed (for kinetic friction, to a good approximation).
The friction force depends strongly on the area of contact between the surfaces.
To a good approximation friction depends on $\mu$ and the normal force, not the contact area.
Friction force is the coefficient $\mu$ times the ____ force.
$f = \mu N$, where $N$ is the normal force pressing the surfaces together.
Select all changes that increase the kinetic friction force on a sliding box.
Friction $= \mu_k N$: a larger normal force or rougher surfaces increase it. Contact area and speed barely matter.
Static friction is not always $\mu_s N$ — that is only its maximum. Below the slipping point, static friction equals whatever push it must balance ($f_s \le \mu_s N$). Also, friction barely depends on contact area, so a wider box is not "grippier".
A box presses on the floor with a normal force $N = 40\ \text{N}$, and $\mu_k = 0.3$.
- While sliding, kinetic friction is $f_k = \mu_k N = 0.3 \times 40 = 12\ \text{N}$.
- This $12\ \text{N}$ opposes the motion, whatever the box's speed.
Static friction ($f_s \le \mu_s N$) grows to prevent motion up to a maximum; kinetic friction ($f_k = \mu_k N$) opposes sliding and is roughly constant. Usually $\mu_s > \mu_k$. Friction $= \mu N$ — it depends on the normal force, not the contact area or speed.