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Momentum & Collisions · Physics

Rocket motion: momentum with mass flowing out.

Rockets accelerate by ejecting mass. Treat it as momentum flow: the system changes mass, so you must account for exhaust leaving the rocket.

This topic

Variable Mass Systems (Rocket Motion)

Learn the core idea (momentum flow), then meet the rocket equation at an intro level.

Concept
Concept of variable mass
The rocket’s mass decreases as fuel is expelled — you can’t treat it like a fixed-mass collision.
  • What changes: mass and momentum
  • Choosing the “system” carefully
  • Why conservation still works (with flow)
Start nano-lesson Think “mass flow.”
Core tool
Momentum flow
Momentum leaves with the exhaust; thrust is tied to exhaust speed and mass flow rate.
  • Exhaust relative speed idea
  • Mass flow rate interpretation
  • Connecting to thrust (conceptual)
Start nano-lesson Build intuition first.
Intro equation
Rocket equation (intro level)
A clean result (under assumptions) linking speed change to exhaust speed and mass ratio.
  • What assumptions are used
  • Meaning of mass ratio
  • How to use it in simple problems
Start nano-lesson Use with care — know assumptions.
Practice
Practice & Exercises
Practice the idea of mass flow and speed change under simplified conditions.
  • Momentum flow concept checks
  • Mass ratio and Δv computations
  • Identify valid assumptions in a problem
  • “Thrust vs speed” conceptual questions
  • Exam-style rocket motion sets (intro)
Open practice sets Keep it conceptual + computational.