AP Physics 1 · Unit 4 · Linear Momentum

Center of Mass

Move masses, locate their shared center of mass, and explain how a system can move predictably while its parts interact.

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Learn by doing

Three ideas you’ll put into motion

Change masses and positions to predict where the center of mass lies.

Relate total momentum to center-of-mass velocity and external force to its acceleration.

Use a separating projectile to compare the parts’ paths with the center-of-mass path.

The big idea

For two objects, x_CM = (m₁x₁ + m₂x₂)/(m₁ + m₂). The result lies closer to the heavier object.

Internal interactions can rearrange the parts without changing total momentum. The net external force determines the center of mass’s acceleration.

A quick comparison

Place masses m and 2m at opposite ends of a light connecting line.

The center of mass lies closer to 2m.

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: Position components, momentum conservation, Newton’s laws, and projectile motion.
Built-in support: Prepared scenes, guided prompts, and feedback on lesson questions.
AP Physics 1 connections: Unit 4: system momentum, center-of-mass motion, and external forces.
Check for understanding: Ask whether an internal explosion alone can change the center-of-mass trajectory.

The first scene is set up. Start the lesson and explore.