WeSolve+ reads the whole document and writes the questions for you
Upload your PDF, photograph your notebook, or point the camera. WeSolve+ writes questions from that material, explains why each answer is right, reads the chapter back to you as a podcast, and remembers every item you missed until you own it.
The tool below is a small browser-only tool and it is not WeSolve+: paste a few lines and text rules turn them into cards on the spot. The real app, the one that uses AI, is behind the link above.
Momentum rules, from your own notes
This is a browser-only tool, and that is all it isIt splits the text you paste by rule, and nothing else. WeSolve+ is a different thing entirely: it reads your whole PDF with AI, writes the reasoning behind every question, speaks the chapter back to you, and remembers what you missed so it can return it. Try the real app now, free!
This tool splits the text you paste by rule and returns cards. It cannot solve a collision or read a graph, so it works on the rules you already wrote down.
Conserved is a claim about the system, not about the universe
Momentum is conserved when no net external force acts on what you chose to call the system. Include both colliding objects and the contact forces become internal and cancel; include only one and they do not. Naming the system in the first line of an answer is the difference between a correct method and a coincidence.
Impulse is the bridge back to forces
Impulse equals the change in momentum and equals the area under a force against time graph. That is why a longer contact time reduces the peak force for the same velocity change, which is the physics behind crumple zones and landing with bent knees. Questions ask for the consequence, not the definition.
Elastic and inelastic differ in energy, never in momentum
Momentum is conserved in both. Kinetic energy is conserved only in an elastic collision. Students who think inelastic means momentum is lost have imported a rule that does not exist, and it produces answers that are wrong in a way the marker can name immediately.
Perfectly inelastic is a gift, so recognise the wording
If the objects move together afterwards they share one final velocity, which removes an unknown and makes the algebra straightforward. Phrases like stick together, embed, or couple are the signal. Missing it means solving a two unknown system that the question did not intend you to face.
Two dimensions means two independent equations
Momentum is a vector, so conservation applies separately along each axis and the components do not mix. Setting up one combined equation is the most common failure in two dimensional collisions, and drawing the axes before writing anything prevents it more reliably than checking afterwards.
What to photograph for Linear Momentum
Free body diagrams, including the wrong ones with corrections. Related: Unit 2, photo to quiz and pricing.
Sources used on this page
- College Board, AP Physics C Mechanics
- Momentum
- Impulse (physics)
- Elastic collision
- Inelastic collision
- Active recall
- Spaced repetition
- Testing effect
- Forgetting curve
- Generation effect
- Judgment of learning
- Metacognition
- Desirable difficulty
- Distributed practice
- Formative assessment
- Flashcard
- Cloze test
- Multiple choice
- Test (assessment)
- Educational assessment
- Advanced Placement
- Curriculum
- Study skills
- Study guide
- Note-taking
- Overlearning
- Instructional scaffolding
- Item analysis
- Mastery learning
| Collision | Momentum | Kinetic energy |
|---|---|---|
| Elastic | Conserved | Conserved |
| Inelastic | Conserved | Not conserved |
| Perfectly inelastic | Conserved | Least conserved of all |
| Explosion | Conserved | Increases, from stored energy |
| External force present | Not conserved | Depends |
| Two dimensions | Conserved per axis | One scalar equation |
What is AP Physics C Mechanics Unit 4?
Linear Momentum: impulse, conservation of momentum, and collisions in one and two dimensions.
When is momentum conserved?
When no net external force acts on the system you chose. Naming the system in the first line is what makes the method correct rather than lucky.
What is impulse?
The change in momentum, equal to the area under a force against time graph. It explains why longer contact reduces peak force.
Is momentum lost in an inelastic collision?
No. Momentum is conserved in every collision without external forces. Kinetic energy is what is not conserved in an inelastic one.
Why is perfectly inelastic easier?
Because the objects share one final velocity afterwards, which removes an unknown. Look for wording like stick together or embed.
How do I handle a two dimensional collision?
As two independent equations, one per axis. Momentum is a vector and the components do not mix, so draw the axes before writing anything.
Last updated: 2026-08-11