Momentum conservation is the principle that the total momentum of a system stays constant when no net external force acts on it, meaning the total momentum of interacting objects before a collision or explosion equals the total momentum after.
Momentum conservation says the total momentum of a closed system never changes. If two carts collide, stick, bounce, or even explode apart, the vector sum of their momenta afterward equals the vector sum before, as long as nothing outside the system pushed on them. That's the whole law, and it falls straight out of Newton's third law. During a collision, object A pushes on B exactly as hard as B pushes on A, so the momentum one gains, the other loses. The internal trade always cancels out.
The catch is the phrase closed system, which is exactly why this idea lives in the Open and Closed Systems topic. A system is closed (for momentum) when the net external force on it is zero. Friction from the floor, a hand pushing a cart, or gravity acting during a long fall can all make a system open, and then total momentum changes. The good news for collision problems is that collisions happen so fast that external forces barely have time to act, so treating the colliding objects as a closed system during the collision is almost always a safe move.
This term anchors Topic 5.3, Open and Closed Systems, where the Fiveable study guide "5.3 Open and Closed Systems: Momentum" covers the full topic. The skill the CED actually wants is system thinking. You define a system, decide whether external forces act on it, and then justify whether momentum is conserved. That justification step shows up constantly in AP scoring guidelines, where "momentum is conserved because the net external force on the system is zero" is the sentence that earns the point. Momentum conservation is also one of the three big conservation laws in the course (alongside energy and angular momentum), so it threads through collisions, explosions, and rotational dynamics. If you can argue from conservation instead of grinding through forces, you solve problems faster and write better FRQ responses.
Keep studying AP Physics 1 Unit 5
Impulse (Unit 5)
Impulse is what breaks momentum conservation. A net external force acting over time delivers an impulse, and that impulse equals the change in the system's total momentum. So conservation is really just the special case where the external impulse is zero. Same equation, J = Δp, read two ways.
Elastic Collision (Unit 5)
In an elastic collision, momentum AND kinetic energy are both conserved. That gives you two equations, which is why elastic problems can pin down both final velocities. Momentum conservation alone usually isn't enough information for an elastic collision.
Inelastic Collision (Unit 5)
Momentum is still conserved in inelastic collisions even though kinetic energy is lost to heat, sound, and deformation. For perfectly inelastic collisions where objects stick together, momentum conservation alone hands you the final velocity in one line.
Torque and Angular Momentum (Unit 5)
Unit 5's rotational content runs on the same logic. Just as zero net external force keeps linear momentum constant, zero net external torque keeps angular momentum constant. If you understand the linear version, the spinning ice skater pulling in her arms is the same argument with rotational vocabulary swapped in.
Multiple-choice questions love before-and-after setups. Two carts collide, an object explodes into pieces, a person jumps off a moving cart, and you compute or compare final velocities using p_before = p_after. Watch for stems that ask whether momentum is conserved, because the answer hinges on identifying external forces and how you defined the system. On FRQs, momentum conservation usually appears inside a multi-part problem, often chained with energy. A classic structure is a collision (use momentum conservation, not energy) followed by the combined object sliding or swinging upward (now use energy conservation). Mixing up which law applies to which phase is the most common way to lose points. You'll also see justification prompts where the credited answer explicitly states that the net external force on the system is zero, so total momentum is constant. Remember momentum is a vector, so 2D problems require conserving x and y components separately.
Momentum is conserved in every collision in a closed system, elastic or inelastic, period. Kinetic energy is only conserved in perfectly elastic collisions. In an inelastic crash, KE converts to heat, sound, and deformation, but the momentum vector total doesn't change at all. On the exam, "is momentum conserved?" and "is kinetic energy conserved?" are two separate questions with potentially different answers, and treating them as the same question is a classic point-loser.
Total momentum of a system is constant whenever the net external force on the system is zero, which is what makes it a closed system for momentum.
Momentum is conserved in ALL collisions and explosions within a closed system, but kinetic energy is only conserved in elastic collisions.
Internal forces between objects in the system never change total momentum, because Newton's third law guarantees they cancel in pairs.
Momentum is a vector, so in 2D problems you conserve the x-component and y-component separately.
On FRQs, earn the justification point by explicitly stating that the net external force on the system is zero, so the system's total momentum is conserved.
In multi-part problems, use momentum conservation during the collision itself and energy conservation for the motion before or after it.
It's the principle that the total momentum of a closed system stays constant. If no net external force acts on a group of interacting objects, the total momentum before a collision or explosion equals the total momentum after. It's the central idea of Topic 5.3 on open and closed systems.
Yes. Momentum is conserved in every type of collision within a closed system. What's NOT conserved in an inelastic collision is kinetic energy, which gets converted to heat, sound, and deformation. Don't confuse losing KE with losing momentum.
Momentum conservation requires zero net external force and holds in all collisions. Kinetic energy conservation only holds in elastic collisions. A standard FRQ chains them, so you use momentum conservation during the collision and energy conservation for the sliding or swinging motion afterward.
When a net external force delivers an impulse to the system, making it an open system. For example, a cart's momentum alone isn't conserved if friction acts on it, but if you redefine the system to include whatever is exerting the force, total momentum is conserved again. System definition is everything.
Collisions are extremely brief, so external forces like friction act for almost no time and deliver a negligible impulse compared to the huge internal collision forces. That's why treating colliding objects as a closed system during the instant of collision is standard practice on the exam.