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Grade 3 · lesson 2 of 4

Count Opposite Force Units

Find the direction and size of an unbalanced horizontal effect.

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Supplied materials

  • Use the supplied fictional observations or model. Paper and pencil are optional; spoken answers are welcome. No equipment or real experiment is needed.

Learn the idea

Opposite forces partly cancel when you combine their effects. To find the remaining effect, subtract the smaller strength from the larger and keep the direction of the larger. Equal opposite forces give zero remaining horizontal effect. “Zero” here means balanced forces, not that no pushes or pulls exist.

Use a paper cart starting at rest. The model lists only two horizontal forces and ignores friction: 7 force units right and 3 force units left. Pair three left units with three right units. Four right units remain. These made-up units let us compare strength; they are a counting model, not measurements of a real cart.

  1. Draw seven right arrow marks and three left arrow marks.
  2. Cross out one opposite pair at a time.
  3. Count and label the four right units that remain.
  4. Repeat on paper for 5 right units and 8 left units.
Worked example

See the method in action

7 right minus 3 left leaves 4 units right. In the second case, 8 left minus 5 right leaves 3 units left. The remaining direction describes the unbalanced horizontal force.

Your turn

Check your reasoning

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A resting model cart has 6 units left and 2 units right. What remains?

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See the answer and explanation

4 units left

Two opposite pairs cancel. Four left units remain because the left force is stronger.

Take it further

Change the right force from 2 to 6 units. The 6-left and 6-right forces now balance, leaving zero horizontal net force.

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These LifeStarter passages, scenarios and exercises are authored for this Academy with AI assistance and review. External resources keep their publishers’ terms. A grade label suggests a starting level; it does not establish alignment with your school’s complete requirements.