Dynamic friction
The block is sliding now. Friction has not gone anywhere — it is still acting, and it still points backwards along the table.
Dynamic friction
The frictional force present once the object has started to move is called dynamic friction. That is, dynamic friction is the frictional force acting against the motion of a moving object.
It is smaller than limiting friction
Keep the balance in view while the block slides steadily and the reading settles a little below the value at which it started moving. Dynamic friction is slightly less than limiting friction.
| Moment | Friction | Which state |
|---|---|---|
| Just before it moves | 18 N | Limiting |
| While it slides | 15 N | Dynamic |
You have felt this
Pushing a heavy cupboard is hardest at the very first instant. Once it is sliding it feels easier to keep going — because friction has dropped from its limiting value to the smaller dynamic one.
A consequence worth noticing
If you go on pulling with the 18 N that started the block, friction is now only 15 N — so 3 N is left unbalanced and the block accelerates. To slide it at constant velocity you have to ease the pull back to 15 N.