AP Physics 1 / Unit 2: Force and Translational Dynamics
2.7 Kinetic and Static Friction
Notes
- Static friction: $f_s\le\mu_sF_N$ (only as much as needed to prevent sliding).
- Kinetic friction: $f_k=\mu_kF_N$ when sliding.
- Usually $\mu_s>\mu_k$.
Practice
A 10 kg box on a floor with $\mu_k=0.3$. Kinetic friction (g = 10 m/s²):
$0.3\times100$.
You push a heavy box with 20 N, and it doesn't move. Static friction:
Matches the push.
It's harder to start a box sliding than to keep it sliding because:
Static friction is larger.
Ready for more? Take the Unit 2 test.