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Grade 3 · lesson 1 of 4

Compare Balanced and Unbalanced Pushes

Use a simple force model to explain whether a cart starts moving.

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Supplied materials

  • Paper and a pencil, or say your answers aloud.

Learn the idea

A force is a push or pull. Forces have directions. When equally strong pushes act in opposite directions on a cart, their horizontal effects balance. A cart that starts at rest can remain at rest when the total horizontal force is balanced.

In our simplified paper model, the only horizontal forces are two opposite pushes. Set A has an equally strong left push and right push. Set B has a stronger right push than left push. The unbalanced effect in B is toward the right. This model leaves out details such as friction, which a real investigation would also consider.

  1. Draw a cart and opposite equal arrows for set A.
  2. Draw a cart with a longer right arrow than left arrow for set B.
  3. Label the carts as starting at rest.
  4. Explain which set has an unbalanced horizontal effect.
Worked example

See the method in action

For set A, equal opposite pushes balance, so the resting cart can stay at rest. Set B has an unbalanced effect toward the stronger right push.

Your turn

Check your reasoning

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In this model, a resting cart has a stronger push to the left than to the right. Which way is its unbalanced horizontal force?

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See the answer and explanation

to the left

The left push is stronger than the opposing right push. The remaining horizontal effect is to the left. Equal opposite forces would balance, but these are unequal.

Take it further

Change the pushes to equal strength. The horizontal force is now balanced. Explain why balanced forces do not mean that no forces are acting.

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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.