College Board-Aligned Original Notes

AP Physics 1: Algebra-Based Unit 4 Topic 3: Conservation of linear momentum

A system's total linear momentum stays constant when the net external impulse on that system is zero.

Unit 4: Linear Momentum. College Board exam weighting listed for this unit: 10%-15% of multiple-choice exam score.

What to Know

Internal forces occur in equal and opposite pairs, so they do not change total system momentum.
Momentum conservation can apply even when kinetic energy is not conserved.
The system boundary determines whether external forces or impulses must be included.
Always connect this topic back to the larger unit: Linear Momentum.

Detailed Notes

Conservation of linear momentum belongs to Unit 4: Linear Momentum. A system's total linear momentum stays constant when the net external impulse on that system is zero. Start by naming the system, choosing axes or a sign convention when needed, and identifying which physical quantity is changing.

Key relationships: Internal forces occur in equal and opposite pairs, so they do not change total system momentum. Momentum conservation can apply even when kinetic energy is not conserved. The system boundary determines whether external forces or impulses must be included.

Model to use: Write total momentum before an interaction equal to total momentum after, using signs for direction. A strong model should include labels, directions, units, and the reason each relationship applies.

AP-style move: State the conservation condition before using the equation: net external impulse is negligible or zero. For free-response work, write the principle first, show the setup, and then interpret the result in the physical context.

Key Vocabulary

Momentum conservation

Constant total momentum for a system with zero net external impulse.

External impulse

Impulse delivered to a system by forces from outside the system.

Internal interaction

A force interaction between objects inside the chosen system.

Explosion event

An interaction where objects push apart while total momentum is conserved.

Recoil

Backward motion caused by momentum conservation after ejecting mass or interacting with another object.

Quick Practice

How would you explain Conservation of linear momentum in one or two AP-style sentences?

Name the concept, apply it to a specific example or source, and explain the reasoning that connects the evidence to your answer.

Official Alignment

Reference and Attribution

AP Physics 1 concept reference: AP Physics 1: Concise Summary Notes by IITian Academy. EduCompanion used this source as a topic checklist and conceptual reference, then rewrote the study notes in original wording.

AP Physics 1: Concise Summary Notes