Balanced forces — Physics, 11–13
When forces cancel, an object does not change its motion. Physics, 11–13 years.
Idea
Several forces can act on one object at the same time. If their pushes and pulls balance, the object stays still or keeps moving at the same speed in the same direction; the forces are still there.
Why
People once saw that a book on a table was not force-free: gravity pulled down while the table pushed up. The idea of balanced forces solves the puzzle of why forces can act without making an object speed up, slow down or turn.
Worked example
A box is pulled right with 12 N and left with 7 N. Draw the arrows, then subtract the smaller force: 12 − 7 = 5 N to the right. The forces are not balanced, so the box changes its motion towards the right.
Common trap
A common mistake is saying that a moving object must have an unbalanced force. That seems reasonable because pushing things usually makes them move, but an unbalanced force is needed to change motion, not simply to keep motion going.
Use
Engineers use balanced forces when checking bridges, shelves and cranes. A bridge may hold a heavy load because downward forces are matched by upward forces; if the balance changes, parts can bend or fail.
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