what would be the momentum of a rocket before flight?
Answers
Explanation:
Rockets provide a wonderful example of Momentum Conservation. As momentum in one direction is given to the rocket's exhaust gases, momentum in the other direction is given to the rocket itself.
First, think of two masses connected by a lightweight (massless!) compressed spring. When the two spring apart, conservation of momentum tells us the Center of Mass remains where it was (or moving as it was).
PTot,i = p1i + p2i = 0 + 0 = 0
PTot,f = p1f + p2f = PTot,i = 0
p1f + p2f = - m1 v1f + m2 v2f = 0
m1 v1f = - m2 v2f
A rocket does much the same thing. Momentum is given to the exhaust gases in one direction so the rocket itself must gain momentum in the other direction. Instead of sending out a single chunk of mass, the rocket fuel is burned continuously. "Continuously" is a code word meaning "use calculus here".
The results are
vf = vi + vexh ln [ Mi / Mf ]
and
Thrust = vexh [ dM / dt ]
Now, for the details, . . .
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Answer:
Before launch, the rocket is at rest on the launch pad, so its momentum is zero. When the rocket engines fire, burning gases are expelled from the back of the rocket. By virtue of the law of conservation of momentum, the total momentum of the rocket and fuel must remain zero. ... As a result, the rocket moves forward.