echoesof.science · Ages 10–11
Balanced and unbalanced forces
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echoesof.science/school/forces-and-motion/ages-10-11/
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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).
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.
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.
Experiment: Fair test: balloon rockets
- 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.
| Pumps of air | Try 1 (cm) | Try 2 (cm) | Try 3 (cm) | Average (cm) |
|---|---|---|---|---|
| 5 | I think the balloon with … pumps will go furthest, because … | |||
| 10 | I think the balloon with … pumps will go furthest, because … | |||
| 15 | I think the balloon with … pumps will go furthest, because … |
Questions
Draw a ball held in your hand. Draw two arrows to show the forces on it. Are they balanced?
🍎Explain why the balloon rocket moves forwards.
In your fair test, what did you change, what did you measure and what did you keep the same?
A football is rolling across the grass and slowly stops. Which forces stop it?
Sources for every fact on this sheet: echoesof.science/school/forces-and-motion/sources/