Static friction
Start with the part of the experiment where nothing appears to happen — the pulls that fail to move the block. Plenty is happening.
Static friction
The friction that acts before motion begins is called static friction. For every applied force smaller than the maximum, an equal and opposite frictional force arises, so the block does not move.
It takes only what it needs
This is the point students most often get wrong. Static friction is not a fixed number waiting to be looked up. It is whatever it has to be.
| You pull with | Friction is | The block |
|---|---|---|
| 4 N | 4 N | stays still |
| 9 N | 9 N | stays still |
| 15 N | 15 N | stays still |
As the applied force grows, the static frictional force acting on the object grows with it. But as the experiment showed, it can only do that up to a certain maximum value. Once the applied force passes that maximum, friction can no longer hold the object at rest, so the object begins to move and picks up a small velocity.
Reading a problem correctly
If a problem says an object is being pushed and is not moving, the frictional force equals the push — not the maximum. The maximum only tells you whether it moves at all.