Whole Earth Curriculum WEC

Lesson 09 — Balanced and Unbalanced Forces

Learners learn that balanced forces (equal and opposite) mean no change in motion, while unbalanced forces mean the motion changes — an object speeds up, slows down, or changes direction. They lay out force arrows, predict, and test with a book, a ball, and a tug-of-war.

D06 P3: Intellectual & Cognitive Awareness D06.S3 50 minutes Draft

Objectives

Essential question

How do balanced and unbalanced forces decide whether an object's motion changes?

forcebalancedunbalancednet forcemotionrestfriction

Materials

Standard materials

Low-tech / no-cost

Enriched / lab & device

Works in different contexts

A diagram of balanced and unbalanced forces. At the top, a book rests on a table with two equal-length arrows, one pushing down labeled 'gravity pulls down' and one pushing up labeled 'table pushes up,' and a caption reads 'balanced forces — equal and opposite, so the book does not move.' Below, a ball on the ground has a long arrow to the right labeled 'kick' and a short arrow to the left labeled 'friction,' with a caption 'unbalanced forces — the kick is stronger, so the ball speeds up.' A third panel shows a cart with two equal opposite arrows labeled 'balanced — steady speed, no change.' Labels, arrow lengths, and shapes, not color alone, carry the meaning so it prints clearly in grayscale.
A diagram of balanced and unbalanced forces. At the top, a book rests on a table with two equal-length arrows, one pushing down labeled 'gravity pulls down' and one pushing up labeled 'table pushes up,' and a caption reads 'balanced forces — equal and opposite, so the book does not move.' Below, a ball on the ground has a long arrow to the right labeled 'kick' and a short arrow to the left labeled 'friction,' with a caption 'unbalanced forces — the kick is stronger, so the ball speeds up.' A third panel shows a cart with two equal opposite arrows labeled 'balanced — steady speed, no change.' Labels, arrow lengths, and shapes, not color alone, carry the meaning so it prints clearly in grayscale.

Lesson 9 — Balanced and Unbalanced Forces

Summary

Learners learn how forces decide motion: balanced forces — equal and opposite — mean the motion does not change, while unbalanced forces mean it does — the object speeds up, slows down, or changes direction. They lay out force arrows, predict, and test with a book, a ball, and a tug-of-war, and they meet the idea of the net force.

Objectives

  • Explain how balanced and unbalanced forces determine whether an object’s motion changes. (D06.S3.06.02)

Connection

Why does a book sit still on a table instead of falling through it? Because two forces are exactly even: gravity pulls it down and the table pushes it up. Why does a kicked ball speed away? Because the kick is stronger than the friction that resists it. From a tug-of-war in a schoolyard to a ferry crossing a river to a rocket leaving the ground, the rule is the same: equal forces mean no change; unequal forces mean change. Today you learn to read the arrows.

Materials

  • Force diagram
  • Force arrow cards
  • Force prediction page

Preparation

  • Print or draw the force diagram, arrow cards, and prediction page.
  • Recall Grade 3: a push or pull (force) changes an object’s motion, and friction resists.
  • Have a book, a ball, and a rope ready for the tests.

Facilitator note

Written to the learner (“you”). The one rule to land: balanced forces = no change in motion; unbalanced forces = motion changes (speed up, slow down, or change direction). The trick is that “balanced” can mean an object at rest or moving at a steady speed — no change either way. Use arrow length to carry size: equal arrows pointing opposite = balanced; unequal = unbalanced, and the net force points the way the longer arrow points. Model the worked example with the book first (gravity down, table up, equal — book stays), then the kicked ball (kick bigger than friction — it speeds up). The tug-of-war is the memorable demonstration. Note that motion at a steady speed with balanced forces is genuinely counter-intuitive — expect learners to think “a force is needed to keep moving”; that is where friction (which they met in Grade 3) explains why things slow down. Keep it concrete. See docs/facilitation.md.

Procedure

  1. Gather (5 min). Look at the book on the table. Why does it not move? And why, when you push it, does it slide and then stop? Write two guesses.
  2. Meet balanced and unbalanced (10 min). A force is a push or pull. When two forces are equal and opposite, they are balanced — the motion does not change. When one is bigger, they are unbalanced — the motion changes, in the direction of the bigger force. Look at the diagram: the book has gravity down and the table up, equal arrows — balanced. The kicked ball has a big kick arrow and a small friction arrow — unbalanced, so it speeds up.
  3. Lay out the arrows (10 min). With your cards, build three cases: two equal opposite arrows (balanced — no change), a long arrow beside a short one (unbalanced — moves toward the long arrow), and a moving cart with equal arrows (balanced — steady speed, no change). For each, say what the motion will do.
  4. Predict and test (15 min). On your page, predict: (a) will a book on the table move? (balanced). (b) will a gently pushed ball speed up, slow down, or keep steady? (unbalanced at first, then friction slows it). Test each with your partner and record what happened and whether the forces were balanced or unbalanced.
  5. Close (5 min). Motion is a tug-of-war: when the pulls are even, nothing changes; when one wins, things move, stop, or turn. Read the arrows and you can predict the motion.

Differentiation

  • Support: Act out the tug-of-war first; use two arrow cards and a “move / no move” verdict for each case.
  • Extension: Draw arrows to scale for a cart pushed two ways at once and find the net force; predict its motion and explain using balanced/unbalanced.

Assessment

  • Formative (observation): Can the learner state the rule (balanced = no change, unbalanced = change) and read a force diagram to predict the motion?
  • Self-check: The learner asks, “For any object, can I draw the arrows and say whether they balance — and what that means for its motion?”

Home connection

Find two forces at home (a door held still, a ball rolling, a bag carried). Tell someone whether the forces are balanced or unbalanced and what the motion shows.

Resources