Grade 3
Forces and Motion
Balanced and unbalanced forces, motion patterns, electric and magnetic interactions, and magnet design problems for Grade 3 learners.
3-PS2-1, 3-PS2-2, 3-PS2-3, 3-PS2-4
4 lessons
100 quiz questions
Lesson 1: Balanced and Unbalanced Forces
Learn how pushes and pulls can start, stop, speed up, slow down, or change an object's direction.
Learning goals
- Describe a force as a push or pull.
- Tell the difference between balanced and unbalanced forces.
- Use everyday examples to explain how forces change motion.
- Use evidence from simple tests to support an idea about forces.
What is a force?
A force is a push or a pull. You use forces all day: you push a door closed, pull a backpack zipper, kick a ball, lift a book, or drag a chair. Forces can be gentle or strong. Forces also have direction. A push to the left is different from a push to the right.
Balanced forces do not change motion
Balanced forces are forces that cancel each other out. Imagine two teams in tug-of-war pulling with the same strength. The rope stays in the middle. The forces are real, but the motion does not change. A book resting on a desk is another example. Gravity pulls the book down, and the desk pushes up. The book stays still because the forces are balanced.
Unbalanced forces change motion
Unbalanced forces do not cancel each other out. If one tug-of-war team pulls harder, the rope moves toward that team. If you kick a still soccer ball, the ball starts moving. If you catch the ball, your hands make it stop. Unbalanced forces can start motion, stop motion, speed something up, slow something down, or change direction.
Try a simple test
Place a toy car on a smooth floor. Give it a tiny push, then a stronger push. Watch how far it travels. To make the test fair, use the same car and the same floor each time. The push strength is the thing you change. The distance traveled is the evidence you collect.
Lesson 2: Motion Patterns
Observe repeated motion and use patterns to predict what may happen next.
Learning goals
- Describe motion as a change in position.
- Identify motion patterns that repeat.
- Use observations and measurements to make predictions.
- Explain why fair tests help us trust motion data.
Motion means changing position
An object is in motion when its position changes. A rolling ball, a moving swing, a falling leaf, and a toy car going down a ramp are all moving. Scientists describe motion by watching where an object starts, where it goes, how fast it moves, and whether it changes direction.
Some motion repeats
A pattern is something that repeats in a way we can describe. A swing moves forward, back, forward, back. A pendulum swings left, right, left, right. A toy train may go around the same track again and again. When motion repeats, we can often predict what will happen next.
Measurements make patterns clearer
Students can use a ruler, tape measure, or timer to collect motion data. For example, you can measure how far a toy car rolls each time it starts from the same ramp height. If the results are similar, you have evidence of a pattern.
Fair tests matter
In a fair test, you change only one thing at a time. If you are testing ramp height, keep the same car, same ramp, and same floor. This helps you know that the ramp height caused the change in motion.
Lesson 3: Electric and Magnetic Interactions
Explore invisible pushes and pulls from magnets and static electricity.
Learning goals
- Identify forces that can act without objects touching.
- Describe attraction and repulsion between magnets.
- Explain how distance and magnet direction can change an interaction.
- Compare magnetic interactions with static electric interactions.
Forces can act without touching
Some forces are contact forces. Your hand must touch a ball to push it. But some forces can act when objects are not touching. A magnet can pull a paper clip through a sheet of paper. A rubbed balloon can pull tiny pieces of paper toward it.
Magnets attract and repel
Magnets have two poles: north and south. Opposite poles attract, which means they pull together. Like poles repel, which means they push apart. If you flip one magnet around, the force can change from pulling to pushing.
Distance can change the force
Magnetic and static electric forces are usually stronger when objects are close together. A magnet may pull a paper clip from 2 centimeters away but not from 20 centimeters away. This is why distance is an important variable to test.
Static electricity
Static electricity can build up when objects rub together. A balloon rubbed on hair may attract hair or tiny paper pieces. The balloon and paper do not need to touch at first. That makes it another example of a force acting at a distance.
Lesson 4: Designing with Magnets
Use magnets to solve simple problems, then test and improve the design.
Learning goals
- Define a simple problem that magnets could help solve.
- Name criteria and constraints for a design.
- Build or draw a magnet-based solution.
- Use test results to improve a design.
Engineers solve problems
An engineer is a person who designs solutions to problems. A solution might be a tool, a model, a machine, a structure, or a process. In this lesson, we use what we know about magnets to solve small problems.
Start with the problem
A good design starts with a clear problem. For example: 'The classroom supply box keeps opening, and we need a way to keep it closed.' Magnets could help because magnetic attraction can hold certain materials together.
Criteria and constraints
Criteria are the goals for a successful design. A magnetic latch might need to keep a lid closed and be easy for a student to open. Constraints are limits, such as using only two magnets, finishing in 20 minutes, or using recycled cardboard.
Test and improve
After building a prototype, engineers test it. If the latch opens too easily, students might move the magnet closer, use a stronger magnet, or add a metal plate. Change one thing at a time so your evidence shows what helped.