Friction with numbers
You push a heavy crate and it does not move. Which force stops it moving?
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- Push 100 N, friction 60 N. The cart speeds up.
- Push 60 N, friction 60 N. The speed stays the same.
- Push 40 N, friction 60 N. The cart slows down.
The force is friction. It pushes back as hard as you push.
The rule
Friction acts against the motion, or against the push when nothing moves. Net force is the push minus the friction. Equal forces leave the motion unchanged.
Forces in the same direction add. Forces in opposite directions subtract. Take 8 N and 5 N. In the same direction they make 13 N. In opposite directions they make 3 N.
At rest, friction matches your push up to a limit. The limit is the most friction the floor can give. Above the limit, the extra push is the net force.
Once it slides, friction is usually a bit less than the limit. To keep the sums simple, this example uses 30 N.
Weight pulls the crate down. The floor pushes up with the normal force. Normal here means at right angles to the floor. On a level floor, the two balance.
| Push | Friction | Net force | Result |
|---|---|---|---|
| 20 N | 20 N | 0 | Stays at rest |
| 30 N | 30 N | 0 | Stays at rest |
| 45 N | 30 N | 15 N | a = 15 ÷ 15 = 1 m/s² |
From rest, its speed after 4 s is 1 × 4 = 4 m/s.