Choose both objects
Identify internal interactions and decide whether external impulse is negligible.
AP Physics 1 · Unit 4 · Linear Momentum
Conservation of Momentum
Choose a system, observe collisions and recoil, and test momentum predictions using the motion of both objects.
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Learn by doing
Three ideas you’ll put into motion
Add signed momenta
Compare the system’s total momentum before and after an interaction.
Test a prediction
Use conservation to predict a collision or recoil result, then check the recording.
The big idea
The key ideas, at a glance.
Conservation depends on the system
Total system momentum stays constant when the net external impulse is zero. Internal forces can change the individual objects’ momenta.
Keep the directions
In one dimension, m1v1i + m2v2i = m1v1f + m2v2f under that condition.
A quick comparison
Two objects push apart from rest with negligible external impulse.
Their momenta are equal in magnitude and opposite in direction.
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
- Momentum, impulse, Newton’s third law, and signed velocity components.
- Built-in support
- Prepared scenes, guided prompts, and feedback on lesson questions.
- AP Physics 1 connections
- Unit 4: conservation of linear momentum and system boundaries.
- Check for understanding
- Ask why equal and opposite internal forces do not require equal final speeds.
- Optional extension
- Investigate the One-Dimensional Collision lab.
Ready to test your thinking?
The first scene is set up. Start the lesson and explore.