Momentum & Collisions · Physics

Explosions and recoil: momentum conservation in action.

Explosions are “time-reversed collisions.” Internal forces can change speeds dramatically, but the system momentum stays the same (if external forces are negligible).

This topic

Explosions & Recoil

Learn to choose the system, use COM thinking, and solve multi-piece breakups cleanly.

Big idea

Explosion as time-reversed collision

Internal forces can be huge, but they come in equal-and-opposite pairs inside the system.

Why momentum can stay constant
What changes: kinetic energy increases
When external forces matter

Classic

Recoil motion

One part goes one way, the other part must “balance” the momentum.

Two-piece breakup in 1D
Recoil speed vs mass ratio
Direction and sign discipline

Power tool

Center-of-mass analysis

The COM frame simplifies breakup problems and highlights symmetry.

COM velocity before and after
Explosions from rest: total momentum = 0
2D breakup and vector momentum

Practice

Practice & Exercises

Turn “explosion” wording into a routine momentum setup.

Breakup from rest (total p = 0)
Moving-object explosion problems
2D recoil + angle sets
COM-frame interpretation questions
Exam-style multi-fragment problems