Rocket and Raven push a big box from opposite sides. Nothing moves.
"Push harder!" Rocket puffs. Raven pushes harder too. Still nothing!
"What do you notice?" Raven asks. "We are both pushing."
Nova hovers above the box. "Two forces can cancel out," she says. "What if one side stops?"
Raven steps back. The box slides toward her!
Sometimes two forces push or pull the same amount in opposite directions.
They cancel each other out. These are balanced forces.
Balanced forces do not change motion.
A still object stays still. A moving object keeps moving the same way.
When forces do not cancel out, they are unbalanced.
Unbalanced forces change motion.
They can make an object start, stop, speed up, slow down or turn.
| What happens | Balanced or unbalanced? |
|---|---|
| Two equal pushes on a box from opposite sides. The box stays still. | balanced |
| A grown-up pushes a bike forward. It starts rolling. | unbalanced |
| A rolling bike slows down when the rider uses the brakes. | unbalanced |
| The handlebars turn and the bike changes direction. | unbalanced |
| Statement | True or false? |
|---|---|
| Balanced forces make a still box start moving. | ? |
| Unbalanced forces can make a bike start rolling. | ? |
| Unbalanced forces can make a bike turn. | ? |
| Balanced forces cancel each other out. | ? |
Brilliant balancing! Tomorrow we meet two forces that are always around a bike: gravity and friction.