School › Forces and motion › Ages 10–11
Balanced and unbalanced forces
Forces have a size and a direction. When they balance, nothing changes; when they don't, things speed up, slow down or turn. With a balloon rocket fair test.
Read
A force is a push or a pull. Every force has a size and a direction. We measure the size of a force in newtons (N).[1][3]
Usually several forces act on an object at once. If they are equal in size and opposite in direction, they cancel out: the forces are balanced. When you hold a ball still, gravity pulls it down and your hand pushes it up with an equal force.[1][4]
When the forces are balanced, an object that is still stays still, and an object that is moving keeps moving at the same speed in a straight line.[2][4]
When the forces are unbalanced, the motion changes: the object speeds up, slows down or changes direction. If you let go of the ball, the forces become unbalanced and it falls.[5][4][1]
Friction and air resistance push against moving things and slow them down. On an air hockey table the puck glides easily, but when the air is turned off it slides only a short way before friction stops it.[2][7]
Forces come in pairs. A rocket pushes gas out backwards, and the gas pushes the rocket forwards. When you jump forwards off a skateboard, the skateboard rolls backwards.[4]
Try it: Fair test: balloon rockets
You need
- Balloons (latex-free if anyone has a latex allergy)
- A long piece of string (about 5 m)
- A drinking straw
- Sticky tape
- Two chairs
- A measuring tape
- A balloon pump (optional, for hygiene)
Safety
Use a pump or your own balloon only; don't share balloons. Check for latex allergies first. Let go of the balloon away from faces. Don't put balloons or burst pieces in your mouth.
What to do
- Thread the string through the straw. Tie the ends of the string to two chairs, about 5 m apart, and pull it tight.
- Blow up a balloon (or use the pump) and hold the neck closed. Do not tie it.
- Tape the balloon to the straw, with its neck pointing towards the start chair.
- Let go and measure how far the balloon travels along the string.
- Fair test: change only ONE thing, the size of the balloon (for example 5, 10 and 15 pumps). Keep the same balloon type, string, straw and start point.
- Do each size three times and work out the average distance.
What happens, and why?
The balloon pushes the air out backwards, and the air pushes the balloon forwards: forces come in pairs. While the push is bigger than friction and air resistance, the forces are unbalanced and the balloon speeds up. When the air runs out, friction from the straw and air resistance slow it down and stop it.[4][5][7]
Play: Balanced or unbalanced?
Look at what the object is doing. Are the forces on it balanced or unbalanced?
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Quiz
Other ages: Ages 6–7 · Ages 15–16