Forces and Motion: Grade 3 Study Guide
A force is a push or a pull. Forces explain why things start moving, stop moving or change direction.
Everything that moves, from a rolling ball to a swinging door, is affected by forces. A push moves an object away from you and a pull brings it closer. Forces can make things start moving, stop, speed up, slow down or change direction.
Scientists describe how strong a force is and which way it points. A bigger push on a swing makes it go higher. A push from the left sends a ball right.
Balanced and unbalanced forces
When forces on an object are balanced (equal and opposite), the object does not change its motion. In a tug-of-war with two equally strong teams, the rope stays still. When the forces are unbalanced, the object moves in the direction of the stronger force. The bigger the difference, the bigger the change.
| Situation | Forces | Result |
|---|---|---|
| Book resting on a table | Gravity down, table pushing up (balanced) | Stays still |
| Tug-of-war, one team stronger | Unbalanced | Rope moves to the stronger team |
| Kicking a soccer ball | Unbalanced | Ball starts to move |
Friction, gravity and magnets
Gravity is a force that pulls objects towards Earth, which is why dropped things fall. Friction is a force that happens when two surfaces rub together and it slows motion; a rough carpet creates more friction than a smooth floor. Magnets can push or pull certain metals (like iron) without even touching them, and two magnets can attract or repel.
Try a fair test
Scientists change one thing at a time to find out what causes an effect.
Does a ramp's steepness change how far a toy car rolls?
Raise the ramp by 5 cm each time.
Same car, same surface, same release point.
Measure the roll distance three times and find the typical result.
Worked examples
Example 1. A box does not move when you push it gently. Are the forces balanced? Yes: your push is matched by friction from the floor, so the box stays still.
Example 2. Two children pull a wagon, one with a strong pull to the right and one with a weak pull to the left. The wagon moves right because the forces are unbalanced.
Example 3. Why does a skateboard slow down and stop when you stop pushing? Friction between the wheels and the ground is an unbalanced force that slows it.
Common mistakes
A common idea is that 'no motion means no forces'. In fact, a still object can have balanced forces acting on it, like gravity and the table pushing up. Another is that heavier objects always fall faster; in air, shape matters too, which is why a flat sheet of paper falls slower than a crumpled one. Finally, remember that magnets work on only some metals, such as iron, not on all metals or on wood and plastic.
Check your understanding: model answers
1. Gravity pulls the child down and the swing's chains pull up. These forces are balanced, so the child is not moving up or down. 2. Carpet has more friction than a smooth floor, so it slows the car more. 3. Roll each ball down the same ramp from the same height, measure the distance three times and compare.
✨ Key Takeaways
- A force is a push or a pull.
- Balanced forces do not change motion; unbalanced forces do.
- Gravity pulls things down; friction slows things; magnets can push or pull without touching.
- A fair test changes one variable at a time.
💬 Questions for Discussion & Reflection
- Describe the forces on a child sitting still on a swing.
- Why does a toy car travel farther on a smooth floor than on carpet?
- Plan a fair test to compare how far two types of ball roll.
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