Conservation of momentum states that the total momentum of an isolated system (no net external force) stays constant before and after any interaction. In AP Physics 2, this rule extends beyond ordinary collisions to photons and matter particles, where momentum connects to wavelength through p = h/λ.
Conservation of momentum is the rule that the total momentum of a system never changes as long as no net external force acts on it. Objects inside the system can push, pull, collide, and explode all they want. Momentum just gets traded between them, and the total before equals the total after. The catch is the phrase isolated system. If an outside force (friction, gravity from outside the system, a hand) pushes on the system, momentum is no longer conserved for that system.
Here's the AP Physics 2 twist. You probably met this law in Physics 1 with carts crashing on tracks. In Topic 7.5, it goes quantum. Photons, even though they have no mass, carry momentum given by p = h/λ, where h is Planck's constant. When a photon hits an electron, momentum is still conserved, exactly like a billiard ball collision. Flip that relationship around and you get the de Broglie wavelength, λ = h/p, which says any object with momentum has a wavelength. Conservation of momentum is the bridge that lets you treat light as a particle and particles as waves with the same bookkeeping.
In AP Physics 2 Revised, conservation of momentum lives in Topic 7.5, Properties of Waves and Particles, inside the modern physics unit. The exam uses it to test wave-particle duality. A photon colliding with an electron is analyzed as a two-body collision where both momentum and energy are conserved, which is exactly the evidence that convinced physicists light behaves like a particle. Momentum is also one of the great conservation laws (alongside energy and charge) that the course expects you to invoke whenever you justify what can or cannot happen in an interaction. If a question asks why an electron recoils when light scatters off it, the answer is momentum conservation, and the photon's momentum comes from p = h/λ.
Keep studying AP Physics 2 Unit 7
de Broglie Wavelength (Unit 7)
The de Broglie relation λ = h/p is just photon momentum run in reverse. Light with wavelength λ carries momentum h/λ, so de Broglie proposed that anything with momentum p has a wavelength h/p. Conservation of momentum is what makes this symmetry physically meaningful instead of just a cute formula.
Planck's Constant (Unit 7)
Planck's constant h is the conversion factor between the wave picture (wavelength, frequency) and the particle picture (momentum, energy). Every momentum calculation for a photon in Topic 7.5 runs through h, since p = h/λ.
Elastic Collision (Unit 7 context)
A photon scattering off a free electron behaves like an elastic collision. Both momentum and kinetic energy are conserved, the photon loses momentum (its wavelength gets longer), and the electron recoils. Same collision physics from mechanics, now at the quantum scale.
Isolated System (cross-unit concept)
Conservation of momentum only applies to isolated systems, meaning no net external force. Defining your system carefully is half the work. A photon plus an electron is isolated during the instant of interaction, which is why the collision analysis works.
On the AP Physics 2 exam, conservation of momentum shows up in modern physics contexts rather than cart-on-a-track problems. Multiple-choice stems give you a photon-particle interaction and ask what happens to the photon's wavelength or the particle's motion afterward, or they ask you to compare the momenta of a photon and a massive particle with the same wavelength. The move you need to make is the same every time. Set total momentum before equal to total momentum after, and use p = h/λ for any photon involved. On free-response questions, conservation laws are the backbone of justification answers. No released FRQ leans on the phrase 'conservation of momentum' by itself in this course, but explaining why a scattered photon's wavelength increases or why an electron recoils requires you to invoke it explicitly, with the system clearly defined as isolated.
Momentum is conserved in every collision within an isolated system, no exceptions. Kinetic energy is only conserved in elastic collisions. In an inelastic collision, momentum still balances perfectly, but some kinetic energy converts to internal energy (think heat and deformation). So if a problem says objects stick together, you can still use momentum conservation, but never set kinetic energy before equal to kinetic energy after.
Total momentum of an isolated system is constant, so momentum before any interaction equals momentum after.
The law only holds when no net external force acts on the system, which is what 'isolated' means.
In AP Physics 2, momentum conservation extends to photons, which carry momentum p = h/λ even though they have zero mass.
The de Broglie wavelength λ = h/p applies the same momentum-wavelength link to matter, giving electrons and other particles a wavelength.
Momentum is conserved in both elastic and inelastic collisions, but kinetic energy is only conserved in elastic ones.
When a photon scatters off a particle, conservation of momentum predicts the photon's wavelength increases and the particle recoils.
It's the law that the total momentum of an isolated system stays the same before and after any interaction. In AP Physics 2 it appears in Topic 7.5, where it governs photon-particle collisions using photon momentum p = h/λ.
Yes. Momentum is conserved in every collision within an isolated system, elastic or inelastic. What's lost in an inelastic collision is kinetic energy, which converts to internal energy, not momentum.
Photon momentum doesn't come from p = mv. It comes from the quantum relation p = h/λ, where h is Planck's constant and λ is the wavelength. Shorter wavelength means more momentum, which is why high-energy photons give particles a bigger kick.
The impulse-momentum theorem (J = Δp) describes how an external force changes one object's momentum over time. Conservation of momentum says the system's total momentum doesn't change when there is no external force. They're two sides of the same idea: no net impulse on the system means no change in total momentum.
Yes, but mostly in modern physics contexts under Topic 7.5. Expect questions about photons scattering off electrons, photon momentum p = h/λ, and the de Broglie wavelength, rather than classic mechanics collision problems, which belong to AP Physics 1.
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