Rocket Propulsion – Law of Action and Reaction A teacher states that Newton’s third law of motion is directly involved when explaining how rockets move in space. Is this statement justified?

Introduction / Context:
Rockets accelerate by expelling mass (exhaust gases) at high speed. Understanding which fundamental law explains this motion is central to basic mechanics and aerospace engineering.

Given Data / Assumptions:

• Rocket ejects exhaust mass backward at high velocity.
• System is isolated over short intervals (neglecting external forces).
• Motion occurs even in vacuum (no need for air to “push against”).

Concept / Approach:
Newton’s third law: for every action, there is an equal and opposite reaction. The rocket exerts a force on the exhaust gases backward; the gases exert an equal and opposite force on the rocket forward. This interaction creates thrust and accelerates the rocket.

Step-by-Step Solution:
Exhaust mass flow rate: ṁ. Relative exhaust speed: u (backward relative to rocket). Thrust magnitude: T ≈ ṁ * u (neglecting pressure terms for simplicity). Direction: forward on rocket, backward on exhaust—an action–reaction pair.

Verification / Alternative check:
Conservation of momentum provides an equivalent explanation: as the rocket imparts backward momentum to exhaust, the rocket gains equal forward momentum. This is consistent with Newton’s third law.

Why Other Options Are Wrong:
“Not justified” or “only near Earth’s surface”: incorrect because rockets operate using internal reaction forces, not relying on air or ground. “Only with aerodynamic lift” or “requires gravity”: thrust arises from mass ejection, independent of atmosphere or gravity.

Common Pitfalls:
Thinking rockets push against air; in vacuum they still accelerate by ejecting mass.

Final Answer:
justified