Three states of one force
Friction is one force, but it behaves in three recognisably different ways depending on the situation. Sorting them out is the whole of this section.
The three cases
- The frictional force acting when a force is applied but no relative motion exists between the bodies.
- The frictional force acting at the moment motion is just about to begin — including the extra force needed to give the object a small velocity.
- The frictional force acting while relative motion is going on between the bodies.
They have names, and the names are worth learning now because the rest of the chapter uses them constantly.
| Case | Name | When |
|---|---|---|
| 1 | Static friction | Pushed, but not moving |
| 2 | Limiting friction | On the point of moving |
| 3 | Dynamic friction | Moving |
An experiment separates the three cleanly, and it needs nothing more than a wooden block and a spring balance. That is the next page.