Find the partner force
Identify the two objects in an interaction and which force acts on each.
AP Physics 1 · Unit 2, Part 1 · Forces
Newton’s Third Law
Inspect pairs of interacting objects and explain why equal and opposite forces can produce very different accelerations.
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Learn by doing
Three ideas you’ll put into motion
Compare the responses
Observe how different masses respond to equal-magnitude interaction forces.
Separate pairs from balance
Distinguish third-law partners from forces that cancel on one object.
The big idea
The key ideas, at a glance.
One interaction, two forces
If A exerts a force on B, B exerts an equal-magnitude force in the opposite direction on A. The forces act simultaneously.
Different objects have different responses
Third-law forces act on different objects. Each object’s acceleration depends on its own mass and net force.
A quick comparison
Two interacting objects have different masses.
Equal interaction forces can produce unequal accelerations.
Teaching this topic?Classroom fit, AP connections, and support
- Classroom fit
- Use for a concept introduction, guided independent practice, or a projected class discussion.
- Before students begin
- Force diagrams, net force, and Newton’s second law.
- Built-in support
- Prepared scenes, guided prompts, and feedback on lesson questions.
- AP Physics 1 connections
- Unit 2: Newton’s third law and interactions between systems.
- Check for understanding
- Ask students to name the partner to the gravitational force exerted by Earth on a book.
- Optional extension
- Try the Newton’s Third Law lab.
Ready to test your thinking?
The first scene is set up. Start the lesson and explore.