Limiting friction
Now the single reading the whole experiment was designed to take: the force at the instant the block just begins to move.
Limiting friction
The maximum frictional force that can arise between the contact surfaces of two touching bodies is called the limiting friction between those two surfaces.
Why the reading gives it
The maximum frictional force the table's surface can create against the block's motion, in the direction opposite to that motion, is equal to the force needed to start the motion. So the balance reading at the instant of starting — 18 N for our block — is the limiting friction.
The two ways of saying it
"The largest friction these surfaces can supply" and "the force needed to just start it moving" are the same number, described from two sides.
Using it to decide
Limiting friction is the number that answers the question will it move?
| Applied force | What happens (limiting friction 18 N) |
|---|---|
| 12 N | Less than 18 N — stays at rest, friction 12 N |
| 18 N | On the point of moving, friction 18 N |
| 24 N | More than 18 N — it moves |
Note that the whole of section 5.3 is about what decides this one number, and the answer turns out to be less obvious than most people guess.