Kinetic and Static Friction
Introduced| English | Meaning |
|---|---|
| static friction/ˈstætɪk ˈfrɪkʃn/ | Friction that opposes a tendency for surfaces in contact to slide relative to each other. |
| kinetic friction/kɪˈnetɪk ˈfrɪkʃn/ | Friction that opposes relative sliding between surfaces in contact. |
| coefficient of static friction/ˌkəʊɪˈfɪʃənt ɒv ˈstætɪk ˈfrɪkʃn/ | The ratio of the maximum friction force to the normal force before sliding starts. |
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?
Difficulty 1/5 · 100 coins + a small bonus on first completion today
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 $\text{N}$?
Difficulty 2/5 · 140 coins + a small bonus on first completion today
$f_k = \mu_k N = 0.3 \times 40 = 12\ \text{N}$.
For most surfaces, how do the coefficients compare?
Difficulty 1/5 · 100 coins + a small bonus on first completion today
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.
Difficulty 1/5 · 100 coins + a small bonus on first completion today
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.
Difficulty 1/5 · 100 coins + a small bonus on first completion today
$f = \mu N$, where $N$ is the normal force pressing the surfaces together.
Select all changes that increase the kinetic friction force on a sliding box.
Difficulty 2/5 · 140 coins + a small bonus on first completion today
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.
Write from a kinetic friction of 15 N
- A sliding box has normal force 60 N and a kinetic-friction coefficient of 0.25.
- Find the kinetic friction.
A sliding box has a normal force of 60 N and a coefficient of kinetic friction of 0.25. What is the kinetic friction?
0.25 times 60 is 15.
What is the kinetic friction?
Difficulty 1/5 · 100 coins + a small bonus on first completion today
0.25 times the normal force of 60 N is 15 N. Other options — 12 N (the lesson value); 60 N (the normal force).