Physics / Unit 3: Forces and Newton's Laws
3.6 Friction
Notes
Friction acts along a surface and opposes sliding (or attempted sliding).
- Static friction holds an object still, up to a maximum $f_s \le \mu_s N$.
- Kinetic friction acts on a sliding object: $f_k = \mu_k N$.
- $\mu$ (the coefficient of friction) has no units and depends on the two surfaces. Usually $\mu_s > \mu_k$, so it is harder to start sliding than to keep sliding.
- Example: a 20 kg box slides on a floor with $\mu_k = 0.30$. $N = 20 \times 9.8 = 196 \text{ N}$, so $f_k = 0.30 \times 196 = 58.8 \text{ N}$.
Practice
A 10 kg sled slides on snow with $\mu_k = 0.10$. What is the kinetic friction force?
$f_k = \mu_k mg = 0.10 \times 10 \times 9.8 = 9.8 \text{ N}$.
A 25 kg box is pulled across a floor at constant velocity with a 49 N horizontal force. What is $\mu_k$?
Constant velocity means friction $= 49 \text{ N}$; $\displaystyle \mu_k = \frac{49}{25 \times 9.8} = 0.2$.
Why is it harder to start pushing a heavy couch than to keep it moving?
Usually $\mu_s > \mu_k$.
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