---
title: "Collision — AP Physics C Mechanics Definition & Exam Guide"
description: "A collision is an interaction where internal forces dwarf external forces, so momentum is conserved. Core to Unit 4 and a favorite in AP Physics C FRQs."
canonical: "https://fiveable.me/ap-physics-c-mechanics/key-terms/collision"
type: "key-term"
subject: "AP Physics C: Mechanics"
unit: "Unit 4"
---

# Collision — AP Physics C Mechanics Definition & Exam Guide

## Definition

In AP Physics C: Mechanics, a collision is a model for an interaction in which the forces the objects exert on each other are much larger than the net external force during the interaction, so the system's total linear momentum is (approximately) conserved even when kinetic energy is not.

## What It Is

A collision is a modeling choice, not just two things bumping. You're claiming that during the brief [interaction](/ap-physics-c-mechanics/unit-2/2-forces-and-free-body-diagrams/study-guide/2LH73zRqxtRXtAKH "fv-autolink"), the forces the objects exert on each other are so much bigger than any outside force ([friction](/ap-physics-c-mechanics/unit-3/2-work/study-guide/Y8X1HNf1OSe29MGP "fv-autolink"), gravity along the motion, a hand pushing) that you can ignore the external stuff. That's the whole justification for writing p_initial = p_final for the system.

The payoff is huge. During a collision the forces are messy, fast, and usually unknown, so Newton's second law alone won't get you anywhere. But because [internal forces](/ap-physics-c-mechanics/key-terms/internal-forces "fv-autolink") come in Newton's-third-law pairs, they cancel inside the system, and total momentum just before equals total momentum just after. Kinetic energy is a different story. In an **elastic** collision kinetic energy is also conserved. In an **inelastic** collision some kinetic energy converts to thermal energy, sound, or deformation. In a **perfectly inelastic** collision the objects stick together and move with one final velocity, which is the maximum possible kinetic energy loss while still conserving momentum.

## Why It Matters

Collisions live in [Unit 4](/ap-physics-c-mechanics/unit-4 "fv-autolink") ([Topic 4.1](/ap-physics-c-mechanics/unit-4/1-linear-momentum/study-guide/vUy5cuBKbgKou3xJ "fv-autolink"), Linear Momentum) and they're the main reason momentum exists as a tool in your kit. The collision model tells you exactly when momentum conservation is valid, which is the kind of justification the exam loves to ask for. It also forces you to keep two conservation laws straight at once. Momentum is conserved in every collision that fits the model; kinetic energy is only conserved in the elastic special case. Mixing those up is one of the most common ways to lose points in this unit. Collisions also act as the bridge in multi-part FRQs, where energy methods get an object TO the collision and momentum conservation gets you THROUGH it.

## Connections

### [Explosion (Unit 4)](/ap-physics-c-mechanics/key-terms/explosion)

An [explosion](/ap-physics-c-mechanics/key-terms/explosion "fv-autolink") is a collision run in reverse. One object (or objects at rest) flies apart, momentum is still conserved, but kinetic energy increases because stored energy (a spring, chemical energy) gets released. Same model, opposite energy bookkeeping.

### Impulse and the Force-Time Graph (Unit 4)

Zoom into the collision itself and [impulse](/ap-physics-c-mechanics/unit-4/2-change-in-momentum-and-impulse/study-guide/dj7haZ7RqOOTuCat "fv-autolink") takes over. The area under the force-versus-time curve equals the change in momentum, which is exactly what the 2018 FRQ tested with a force sensor on a cart. Conservation of momentum tells you the before-and-after; impulse tells you what happened in between.

### Conservation of Energy (Unit 3)

Classic FRQ structure, like the 2019 [pendulum](/ap-physics-c-mechanics/key-terms/pendulum "fv-autolink") problem, chains the two laws. Use energy conservation for the swing down, switch to momentum conservation for the collision at the bottom, then switch back to energy for whatever happens after. Knowing where to switch tools is the skill being tested.

### Angular Momentum Collisions (Unit 6)

The same model returns in rotation. When a moving object strikes a rod on a pivot (like the 2023 FRQ), linear momentum is not conserved because the pivot exerts an external force, but angular momentum about the pivot is. The collision idea generalizes; you just swap which quantity survives.

## On the AP Exam

Collisions show up almost every year, in both MCQs and FRQs. MCQ stems give you masses and velocities and ask for a final velocity (perfectly inelastic, like a bullet embedding in a block, or elastic in one dimension) or ask which quantities are conserved. FRQs are where the model really gets tested. The 2018 FRQ used a force sensor during a cart collision, linking impulse to momentum change. The 2019 FRQ chained energy conservation (pendulum swing) into momentum conservation (collision at the bottom). The 2022 FRQ had carts colliding after one rolled down an incline. The 2023 FRQ moved the idea into rotation with a rod-sphere collision conserving angular momentum. You need to do three things: justify why momentum is conserved (internal forces dominate external forces), classify the collision to decide whether kinetic energy is conserved, and know when to switch between energy and momentum methods.

## collision vs explosion

Both are interactions where internal forces dominate, so momentum is conserved in both. The difference is the direction of the energy flow. In a collision, kinetic energy stays the same (elastic) or decreases (inelastic). In an explosion, kinetic energy increases because stored potential energy, like a compressed spring between two carts, converts into motion. If the pieces fly apart faster than they came together, you're looking at an explosion.

## Key Takeaways

- A collision is a model that applies when the forces between the objects are much larger than the net external force, which is what justifies using conservation of momentum.
- Total linear momentum of the system is conserved in every collision that fits the model, regardless of whether the collision is elastic or inelastic.
- Kinetic energy is conserved only in elastic collisions; in inelastic collisions some kinetic energy converts to thermal energy or deformation.
- In a perfectly inelastic collision the objects stick together, share one final velocity, and lose the maximum possible kinetic energy while still conserving momentum.
- Multi-part FRQs often require switching tools, using energy conservation before and after the collision but momentum conservation through it.
- The same collision logic extends to rotation in Unit 6, where angular momentum about a pivot is conserved even when linear momentum is not.

## FAQs

### What is a collision in AP Physics C Mechanics?

It's a model for an interaction where the forces between the objects are much larger than the net external force, so the system's total linear momentum is conserved during the interaction. It covers everything from carts bumping to a bullet embedding in a block.

### Is momentum always conserved in a collision?

Yes, as long as the situation fits the collision model where internal forces dominate external forces. That's true for elastic, inelastic, and perfectly inelastic collisions alike. What changes between types is kinetic energy, not momentum.

### Is kinetic energy conserved in an inelastic collision?

No. In an inelastic collision some kinetic energy converts to thermal energy, sound, or deformation. Only elastic collisions conserve kinetic energy, and perfectly inelastic collisions (objects stick together) lose the maximum amount possible.

### What's the difference between a collision and an explosion?

Both conserve momentum, but energy flows in opposite directions. A collision keeps or loses kinetic energy, while an explosion gains kinetic energy from a stored source like a spring or chemical energy. Two carts pushed apart by a spring is an explosion, not a collision.

### How do I solve a perfectly inelastic collision problem?

Set total momentum before equal to total momentum after, with the objects sharing one final velocity: m₁v₁ + m₂v₂ = (m₁ + m₂)v_f. For example, a 0.15 kg bullet embedding in a 5.0 kg block on a frictionless surface is the classic setup, and you solve directly for v_f.

## Related Study Guides

- [4.1 Linear Momentum](/ap-physics-c-mechanics/unit-4/1-linear-momentum/study-guide/vUy5cuBKbgKou3xJ)

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