The three states in one graph
Everything in this section fits on a single graph: the frictional force f plotted against the force F applied to the block.
| F (N) | 18 |
|---|---|
| f (N) | 18 |
Reading it
- The 45° rise. Up to 18 N the two are equal — friction gives back exactly what is applied, so the line runs at 45°. This whole stretch is static friction, and along all of it the block is at rest.
- The peak. The dot at 18 N is the limiting friction, the most these two surfaces can supply. The block is on the point of moving.
- The drop and the flat line. Past the peak the block slides, friction falls to the dynamic value of 15 N, and it stays there no matter how much harder you pull.
| When it acts | Its size | The object | |
|---|---|---|---|
| Static | Force applied, no motion | Equal to the applied force | At rest |
| Limiting | Motion just beginning | The maximum possible | About to move |
| Dynamic | While sliding | A little less than limiting | Moving |
The one thing to carry away
Friction is not a single number belonging to a pair of surfaces. Which of the three you are dealing with is decided by what the object is doing, and that is the first thing to establish in any problem.