Momentum is the product of an object's mass and velocity (p = mv), a vector quantity that measures how hard it is to change an object's motion. In AP Physics 2, it's conserved in all collisions and is the key to understanding elastic collisions between gas molecules in thermodynamics (Topic 2.8).
Momentum is mass times velocity, written p = mv. It's a vector, so direction matters. A bowling ball rolling slowly can have the same momentum as a baseball flying fast, and that's exactly the point. Momentum captures how much "oomph" a moving object carries and how hard it is to stop.
You met momentum in AP Physics 1, but Physics 2 puts it to work in a new setting. In Topic 2.8, gas molecules are modeled as tiny particles undergoing elastic collisions with each other and with container walls. Every one of those collisions conserves momentum. When a molecule bounces off a wall, its momentum changes direction, and that momentum transfer to the wall, repeated trillions of times per second, is what we measure as gas pressure. Momentum is the microscopic machinery behind a macroscopic thermodynamic quantity.
Momentum lives in Topic 2.8, Thermodynamics and Elastic Collisions, where the kinetic theory of gases depends on it completely. The whole model of an ideal gas assumes molecules collide elastically, meaning both momentum and kinetic energy are conserved in every collision. That assumption is why gas molecules don't gradually slow down and why temperature stays constant in an isolated gas. Momentum also bridges into modern physics later in the course, where photons carry momentum despite having no mass and matter has a wavelength tied to its momentum. So this one concept connects Newtonian mechanics, thermodynamics, and quantum behavior, which is exactly the kind of cross-unit reasoning the exam rewards.
Keep studying AP Physics 2 Unit 2
Conservation of Momentum (Unit 2, Topic 2.8)
In any collision with no outside forces, total momentum before equals total momentum after. This is the rule that lets you predict what happens when two gas molecules collide, and it holds whether the collision is elastic or not.
Elastic Collision (Unit 2, Topic 2.8)
An elastic collision conserves both momentum AND kinetic energy. Kinetic theory assumes gas molecule collisions are elastic, which is why a sealed gas at constant temperature never loses energy to its own internal collisions.
Kinetic Energy (Unit 2)
Momentum and kinetic energy are teammates, not twins. Both depend on mass and velocity, but momentum is a vector (mv) and kinetic energy is a scalar (½mv²). In thermodynamics, average kinetic energy of molecules sets the temperature, while their momentum transfers set the pressure.
Impulse (Unit 2)
Impulse is the change in momentum caused by a force acting over time. When a gas molecule rebounds off a container wall, the wall delivers an impulse that flips the molecule's momentum, and by Newton's third law the molecule pushes back on the wall. Add up all those tiny pushes and you get gas pressure.
Expect momentum to show up as the reasoning engine behind other answers rather than as a standalone definition question. Multiple-choice stems ask you to explain gas pressure microscopically (momentum transfer per wall collision), compare what's conserved in elastic versus inelastic collisions, or predict molecule speeds after a collision. On the free-response side, the particle model of light leans on momentum too. The 2021 short FRQ on modeling light as waves or particles is the kind of question where photon momentum justifies particle-like behavior. Your job is rarely just to compute p = mv. It's to argue, in writing, that momentum is conserved and use that fact to explain a physical outcome.
Momentum (p = mv) is a vector and is conserved in EVERY collision. Kinetic energy (½mv²) is a scalar and is only conserved in elastic collisions. Two objects can collide, stick together, and lose kinetic energy to heat while total momentum stays exactly the same. If an exam question asks what's always conserved in a collision, the answer is momentum, not kinetic energy.
Momentum equals mass times velocity (p = mv) and is a vector, so direction matters when you add momenta together.
Total momentum is conserved in every collision when no external forces act, whether the collision is elastic or inelastic.
Elastic collisions conserve both momentum and kinetic energy, which is the core assumption of the kinetic theory of gases in Topic 2.8.
Gas pressure is the macroscopic result of countless molecules transferring momentum to container walls during elastic collisions.
Photons carry momentum even though they have no mass, which is part of the evidence for the particle model of light in modern physics.
Momentum is an object's mass multiplied by its velocity (p = mv), a vector that measures how hard it is to change the object's motion. In AP Physics 2 it explains gas pressure through molecule-wall collisions and stays conserved in every collision.
Yes. Momentum is conserved in all collisions as long as no external force acts on the system. It's kinetic energy that can be lost (to heat, sound, or deformation) in inelastic collisions, not momentum.
Momentum is mv and has direction; kinetic energy is ½mv² and doesn't. Momentum is conserved in every collision, while kinetic energy is conserved only in elastic ones. That distinction is one of the most commonly tested ideas tied to this term.
Each time a gas molecule bounces elastically off a container wall, its momentum reverses, which means the wall received an impulse. Trillions of these momentum transfers per second add up to a steady force per area, and that's pressure.
Yes. Photons carry momentum related to their energy and wavelength even though they're massless, which is why p = mv doesn't apply to them. This is part of the particle model of light, the kind of reasoning tested on the 2021 short FRQ about modeling light as waves or particles.