Balanced and unbalanced forces
We can now answer precisely why the table did not move. The force applied and the friction cancelled each other out.
Balanced forces
If the force we apply is small, it comes into balance with the resisting force.
The total force on the object is then zero, so it does not move.
Unbalanced forces
Once enough force is applied to move it, the resisting force can no longer balance it.
An unbalanced force is then left over, and the object begins to move.
The rule
On any occasion when an unbalanced force acts on an object that is at rest, that object begins to move.
The bus again
| Situation | Force applied | Result |
|---|---|---|
| One person | Less than the friction | Forces balanced — no motion |
| A group | More than the friction | Force unbalanced — the bus moves |
Same bus, same friction — the only thing that changed is the force applied.
A misunderstanding to avoid
"No force" and "balanced forces" are not the same thing. While the bus is being pushed there are two forces at work, and they cancel each other out. Only the visible result is the same.