Static vs Kinetic
Static vs Kinetic Friction
When you push a resting crate, friction first matches your effort and keeps it still.
Once the crate slides, a weaker but steady friction resists its motion.
Key Points:
- Static friction: variable, equals applied force up to \(f_s^{\text{max}}\).
- Kinetic friction: constant \(f_k\) after motion starts, and \(f_k < f_s^{\text{max}}\).
- Direction: always opposite to intended or actual motion.
Friction vs Load
Graph of limiting static friction (Flim) versus normal force (N)
Graphical insight
The straight line starts at the origin, so limiting static friction \(F_{\text{lim}}\) is zero when normal force \(N\) is zero.
Slope of the line equals coefficient of static friction: \( \mu_s = \frac{F_{\text{lim}}}{N} \).
Key Points:
- Linear graph shows direct proportionality \(F_{\text{lim}} \propto N\).
- Double \(N\) ⇒ double \(F_{\text{lim}}\).
- Gradient gives \( \mu_s \), a constant for the contact pair.