Balanced and unbalanced forces — Physics, 7–10
Several pushes or pulls can work together. When they balance, motion does not change; when they do not, motion changes.
The forces can balance
A force is a push or a pull, and more than one force can act on the same object. If opposite forces are equal, they balance: a still object stays still, or a moving object keeps the same speed and direction. If one side is stronger, the motion changes.
Why this matters
At first, it seems that any force must make something move. But a table can hold a book without the book falling, because the table pushes up as strongly as Earth pulls down. Comparing all the forces explains why an object stays put or starts changing its motion.
A tug-of-war
Two children pull a rope in opposite directions. One pulls with 40 newtons and the other with 40 newtons, so the total force is 40 − 40 = 0 newtons: the rope does not speed up either way. If one child pulls with 50 newtons, the difference is 10 newtons towards that child, so the rope starts moving that way.
The balanced-force mistake
A common mistake is saying, “There are no forces because nothing moves.” That is reasonable: we often notice motion more easily than support. In fact, a parked bicycle still has Earth’s pull downward and the ground’s push upward. The forces are present; they simply cancel.
Where it appears
Balanced forces help buildings, shelves and bridges remain steady while they carry loads. Engineers check that the forces on each part fit together safely. When a football is kicked, the balance is broken for a moment, so the ball speeds up and changes direction.
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