Balanced and unbalanced forces
Physics · Key Stage 2 · The School
Two forces, pulling against each other
Push a heavy box and nothing happens at first. Your push is a force. Friction pushes back just as hard. The two forces are BALANCED — equal and opposite — so the box stays still. Push harder and your force wins. Now the forces are UNBALANCED, and the box slides.
Spotting balanced forces
A book resting on a table looks like nothing is happening. But two forces act on it. Gravity pulls it down. The table pushes back up, just as hard. These forces are balanced, which is exactly why the book stays still instead of falling through the table or floating away.
What unbalanced forces do
When forces are unbalanced, something always changes. A moving object speeds up, slows down, or changes direction. A still object starts moving. The bigger the difference between the two forces, the bigger the change. A gentle unbalanced push causes a small change. A hard one causes a big change.
Two teams pull a rope in opposite directions. If both teams pull with the same force, the forces are balanced and the rope stays still, even though everyone is pulling hard. One team pulls harder. Now the forces are unbalanced. The rope moves towards the stronger team, even though the weaker team has not stopped pulling at all.