S3.7.3 For the adult
Balanced and unbalanced forces
Why does a bag of rice on a table not move, when gravity is pulling it down?
What this chapter teaches
Forces in opposite directions are taken one from the other to give the overall force. When the overall force is zero the forces are balanced, and a moving thing keeps its speed and direction.
Three questions to ask
There is no one right wording. Listen for the idea.
- A box is pushed with 25 N and friction pulls back with 10 N. What is the overall force?
- Why does a bag on a table stay still when gravity pulls on it?
- Does a moving thing need a force to keep it moving? What changes its speed?
After the chapter, the learner can
- Draw force arrows on an object and find the overall force
- Explain why balanced forces do not change speed or direction
What they copy into their notebook
Forces in opposite directions: take one from the other. That is the overall force. Balanced forces give zero, so speed and direction do not change.