🧭 Plot Summary
Friction isn't one force — it's two, and which one applies depends on whether an object is already sliding. Static friction acts on objects that aren't moving yet, and it's clever: it automatically matches whatever force you apply, right up until it hits a maximum value it can't exceed. Push harder than that maximum, and the object breaks loose.
Once it's sliding, static friction is done — kinetic friction takes over, and it's simpler: a roughly constant force while the object keeps moving. Kinetic friction is usually smaller than static friction's maximum, which is exactly why a heavy box feels like it "gives" the instant it starts sliding.
What you will do in this lesson
- Meet two different frictions hiding behind one everyday word.
- Learn why it always takes more force to get something moving than to keep it moving.
- Use f_s ≤ μsN and f_k = μkN to predict whether an object slides at all.
- Read the applied-force-vs-friction graph that explains the sudden 'give' when something breaks loose.
- Practice drawing friction correctly on a free-body diagram — direction matters as much as size.
Why it matters
Project 2-2-4: Friction Ramp Lab asks you to measure a real coefficient of friction and predict the exact angle where an object starts to slide. None of that works without today's distinction between static and kinetic friction locked in first.
✅ Self-Check Before You Roll On
Check off each item as you get there. These are not grades — they are your own signal.