BuilderGrade 3⏱ 12 min
Balanced and unbalanced forces
Play tug of war: equal pulls cancel out, and a bigger pull makes things move.
Standards: 3-PS2-1
1Wonder
In tug of war, both teams pull as hard as they can, but sometimes the rope does not move at all. Why?
2Explore
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Turn your phone sideways, or open this lesson on a tablet or computer. The explanation and quiz below work here.
Red team 200 N Blue team 200 N Total force 0 N (none) Each kid pulls with 100 N (newtons).
Red team (kids)
Blue team (kids)
Try this
- Make both teams the same size and press Pull! What happens?
- Give the blue team one more kid. Who wins now?
- Try 4 against 1, then 3 against 2. Which rope moves faster?
- What is the total force when both teams have 3 kids?
3Explain
Step 1 of 5
A force is a push or a pull
Pushing a box and pulling a wagon are both forces. A force can start something moving, stop it, or change its direction.
Total force
Total force = bigger force − smaller force (when they point in opposite directions).
- Total force 0 → balanced → no change in motion.
- Total force not 0 → unbalanced → the motion changes.
4Practice
Question 1 of 5
What will the box do?
5Recap
- A force is a push or a pull. It has a size and a direction.
- Balanced forces are equal and opposite. They cancel, so nothing starts moving.
- Unbalanced forces make an object start moving, speed up, slow down or turn.
- To find the total force, subtract forces that point in opposite directions.