What friction is
The table gave us the behaviour. Here is the definition that captures it — worth reading twice, because every word in it is doing a job.
Frictional force
When there is a tendency for a relative displacement to occur between two bodies in contact with each other, or when such a relative displacement is already taking place, the forces acting between the surfaces of those two bodies to prevent that displacement are called frictional forces.
Reading it slowly
| The words | What they are guarding against |
|---|---|
| in contact with each other | No touching, no friction. It is not a force that acts at a distance the way gravity does. |
| a tendency for displacement | Friction is there even when nothing is moving. The gently pushed table was not moving, yet friction was acting on it. |
| or already taking place | And it is still there once the object slides. It does not switch off at the moment motion starts. |
| to prevent that displacement | This fixes its direction. Friction always opposes the motion, or the attempted motion — never helps it along. |
A common slip
"Friction acts only on moving objects" is wrong, and the definition is careful to say so. A crate sitting still on a slope, a book you are pressing sideways against a wall, the table you pushed too gently — friction is at work in all three.