Physics · 9th Grade

Active learning ideas

Newton's Third Law: Action and Reaction

Active learning works for Newton’s Third Law because students often struggle to see equal-but-opposite forces in real time. Hands-on activities let them feel, measure, and visualize paired forces instead of just hearing about them. This builds the intuition needed to spot action-reaction pairs in everyday motions like walking or collisions.

• Common Core State Standards
• HS-PS2-1
• HS-PS2-3

• 20–35 min
• Pairs → Whole Class
• 4 activities

Activity 01

• Inquiry Circle
• 30 min · Pairs

Inquiry Circle: Dual Force Sensors

Pairs connect two digital force sensors and pull, push, tap, and jerk them against each other in different ways while watching the real-time display. They observe that the magnitude readings are always identical regardless of who is applying force or how forcefully.

If forces always occur in equal and opposite pairs, how does anything ever move?

Facilitation Tip

During Collaborative Investigation: Dual Force Sensors, have students share their graphs with the class so everyone sees the paired peaks and valleys in real time.

What to look for

Present students with an image of a person jumping off a skateboard. Ask them to: 1. Identify the action force. 2. Identify the reaction force. 3. Explain why the person moves forward and the skateboard moves backward.

• Analyze
• Evaluate
• Create
• Self-Management
• Self-Awareness

Activity 02

• Role Play
• 25 min · Whole Class

Role Play: Skateboard Collisions

Students stand on skateboards or rolling chairs and push off each other in pairs of different masses. The class records directions and relative speeds, identifies the interaction pair in each push, and explains why the lighter student moves faster using Newton's Second Law.

How does a rocket engine produce thrust in the vacuum of space?

Facilitation Tip

In Role Play: Skateboard Collisions, stand to the side and narrate the forces aloud as students act them out to reinforce the equal-and-opposite pairs.

What to look for

Pose the question: 'A book rests on a table. What is the action force, and what is its reaction force?' Guide students to identify that the book exerts a force on the table, and the table exerts an equal and opposite force on the book, not that the book's weight is cancelled by the table's support force.

• Apply
• Analyze
• Evaluate
• Social Awareness
• Self-Awareness

Activity 03

• Think-Pair-Share
• 20 min · Pairs

Think-Pair-Share: Why Does Anything Move?

Pairs discuss the apparent paradox that if the road pushes the car forward with the same force the car's tires push back on the road, why does the car accelerate. Students must articulate that the paired forces act on different objects and apply F = ma to each object separately.

What is the interaction pair for the weight of a book sitting on a table?

Facilitation Tip

For Think-Pair-Share: Why Does Anything Move?, circulate while pairs discuss and deliberately ask one group to share their free-body diagrams on the board.

What to look for

Provide students with a scenario: 'A cannon fires a cannonball.' Ask them to: 1. Draw a diagram showing the cannon and cannonball, and label the forces they exert on each other. 2. Briefly explain why the cannon recoils much less than the cannonball moves forward.

• Understand
• Apply
• Analyze
• Self-Awareness
• Relationship Skills

Activity 04

• Simulation Game
• 35 min · Pairs

Simulation Game: Rocket Engine Design

Using a virtual rocket simulator, students adjust exhaust mass flow rate and velocity to achieve a target thrust value. They must connect the force on the expelled gases (action) to the reaction force on the rocket body and explain why the rocket accelerates in a vacuum with nothing to push against.

If forces always occur in equal and opposite pairs, how does anything ever move?

Facilitation Tip

During Simulation: Rocket Engine Design, pause the simulation after each trial and ask students to predict the next outcome based on force magnitudes and mass differences.

What to look for

Present students with an image of a person jumping off a skateboard. Ask them to: 1. Identify the action force. 2. Identify the reaction force. 3. Explain why the person moves forward and the skateboard moves backward.

• Apply
• Analyze
• Evaluate
• Create
• Social Awareness
• Decision-Making

A few notes on teaching this unit

Experienced teachers approach Newton’s Third Law by first letting students experience the forces before naming them. Avoid starting with formal definitions; instead, begin with concrete collisions or pushes so students feel the recoil or resistance. Research shows that asking students to predict outcomes before measuring (predict-observe-explain) sharpens their attention to the paired forces and reduces confusion about cancellation.

By the end of these activities, students should consistently draw two separate free-body diagrams for interacting objects and correctly label equal-magnitude forces acting on different bodies. They should also explain why equal forces don’t cancel and connect force pairs to observable motion changes.

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Watch Out for These Misconceptions

• During Collaborative Investigation: Dual Force Sensors, watch for students who think the force readings should be different when one sensor is held by a larger student or pushed harder.

  Have students pause and look at both sensor displays simultaneously. Ask them to note that the peaks occur at the same time and have the same numerical value, regardless of who holds the sensor or how hard they push. Emphasize that the sensors measure force on each object, not the effort of the person pushing.

• During Role Play: Skateboard Collisions, watch for students who believe the larger person exerts more force on the smaller person during a collision.

  After the role play, point to the marked ‘force pairs’ on the floor and ask students to compare the pushes they felt. Then, use the dual force sensors to show equal force readings when two different-mass carts collide, connecting the felt push to the measured force.

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Methods used in this brief

• Inquiry Circle
• Role Play
• Think-Pair-Share
• Simulation Game