In AP Physics 1, an explosion is a model for an interaction in which forces internal to a system push the system's objects apart. Because the forces are internal, the system's total momentum (and its center-of-mass velocity) is unchanged by the explosion.
An explosion is one of the two big interaction models in Unit 4 (the other is a collision). The CED defines it as an interaction in which forces internal to the system move objects within that system apart. A firework shell bursting, a spring launching two carts in opposite directions, an astronaut throwing a wrench. Physically these look different, but to the AP exam they're all the same model.
The punchline is that internal forces can't change a system's total momentum. The pieces push on each other with equal-and-opposite forces (Newton's third law), so every momentum gain by one fragment is exactly canceled by the others. If the net external force on the system is zero, total momentum before the explosion equals total momentum after, and the center of mass keeps moving exactly as it did before. An explosion is basically a collision run backwards: instead of objects coming together, they fly apart, but the momentum bookkeeping is identical.
Explosions live in Unit 4 (Linear Momentum), specifically Topics 4.1 and 4.3. Learning objective 4.1.A names explosions directly as one of the interactions momentum is used to analyze, and 4.3.A and 4.3.B are where the real work happens. 4.3.B is the system-selection skill, which is the whole game here. If you choose the system to include all the fragments, the explosive forces are internal, the net external force is zero, and momentum is conserved. If you choose just one fragment, that fragment's momentum absolutely changes because the other pieces exert external forces on it. The exam loves testing whether you can make that distinction. Explosions are also the cleanest setting for the center-of-mass idea in 4.3.A, because a system can blow into a dozen pieces and its center of mass keeps cruising along the original trajectory like nothing happened.
Keep studying AP® Physics 1 Unit 4
Conservation of Linear Momentum (Unit 4)
Explosions are the showcase problem for 4.3.A and 4.3.B. Set total momentum before equal to total momentum after, and you can solve for an unknown fragment's velocity even when the pieces fly off in two dimensions.
Collisions and the object model (Unit 4)
A collision and an explosion are mirror images. In a collision, objects come together; in an explosion, internal forces drive them apart. Both use the object model, meaning you only compare the initial and final states and skip the messy details in between.
Center-of-mass velocity (Unit 4)
With no net external force, the center of mass of an exploding system keeps its velocity. A firework shell that bursts at the top of its arc has fragments scattering everywhere, but the center of mass still follows the original projectile path.
Newton's Third Law (Unit 2)
The reason explosions conserve momentum is third-law force pairs. Each fragment pushes on the others with equal-magnitude, opposite-direction forces, so the impulses inside the system cancel out perfectly.
Explosion problems show up in multiple-choice and free-response, and they almost always test the same core moves. You'll be asked to (1) find an unknown fragment's velocity by setting total momentum before equal to total momentum after, often in two dimensions where x and y momentum are conserved separately, (2) state what happens to the center-of-mass velocity (it doesn't change if net external force is zero), and (3) justify your answer using system selection. A classic stem gives you a 3.0 kg object exploding into three fragments, hands you two fragment velocities at right angles, and asks for the third. Another favorite is the firework bursting at the peak of its trajectory, where the answer hinges on recognizing the center of mass continues on the original parabolic path. No released FRQ has leaned on the word 'explosion' by itself, but conservation-of-momentum reasoning with system justification is bread-and-butter FRQ material, so be ready to write 'the forces are internal to the system, so total momentum is conserved' as part of a paragraph-length argument.
Both are Unit 4 interaction models analyzed with conservation of momentum, and both compare only initial and final states. The difference is direction and energy. In a collision, objects come together and kinetic energy either stays the same (elastic) or decreases (inelastic). In an explosion, internal forces push objects apart and the system's kinetic energy increases, because stored energy (chemical, spring, etc.) gets converted into motion. Momentum is conserved in both; kinetic energy behaves oppositely.
An explosion is a model for an interaction where forces internal to the system push the system's objects apart.
Because the forces are internal, an explosion cannot change the system's total momentum; momentum before equals momentum after.
The center-of-mass velocity of an exploding system is unchanged as long as the net external force on the system is zero.
Kinetic energy increases in an explosion (stored energy becomes motion), even though momentum is conserved, which is the opposite of an inelastic collision.
Whether momentum is conserved depends on your system choice: include all fragments and momentum is conserved, but a single fragment's momentum definitely changes.
In 2D explosion problems, conserve the x-component and y-component of momentum separately.
It's a model for an interaction in which internal forces within a system move the objects of that system apart, like a firework bursting or a spring separating two carts. Since the forces are internal, the system's total momentum is unchanged.
Yes, as long as your system includes all the fragments and the net external force is zero. The explosive forces are internal third-law pairs, so they cancel and total momentum stays exactly what it was before the blast.
No. Kinetic energy increases because stored energy (chemical energy in gunpowder, elastic energy in a spring) is converted into motion of the fragments. This is the big contrast with collisions, where kinetic energy stays the same or decreases.
A collision is objects coming together; an explosion is objects flying apart due to internal forces. Both conserve total momentum and both are analyzed by comparing only initial and final states, but kinetic energy decreases or stays constant in collisions and increases in explosions.
Nothing changes. If the net external force is zero, the center of mass keeps its velocity, so a 4.0 kg firework shell that explodes at the top of its trajectory has a center of mass that continues along the original projectile path.
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