# Homework Help: Rocket/gas question

1. Feb 20, 2006

### physicsilliterate69

I have a question on a homework assignment and I am kind of stuck.

A big rocket at launch has a huge ball of flaming gas that is deflected by the ground at the beginning. Does the gas hitting the ground have anything to do with the rocket taking off? Think carefully here---is this really any different than the ship leaving the station---was there anything to "push against" there?

I know that the gas disappates or speads out once it leaves the confinement of the rocket. Basically the gas molecules can move around a bigger space (not confined). The gas hitting the ground pushes against the earth but I don't think that it has anything to do with the rocket taking off. But, then again, the gas is pushing against the earth and the earth is pushing against the gas. The gas spreads outward as the earth will not allow it to push downward into the earth.I'm getting confused at this point.

Anyone out there can explain this better to me??

2. Feb 20, 2006

### Janus

Staff Emeritus
Try thinking about it this way. Would the recoil of a gun be any different if the bullet hit something shortly after leaving the gun as opposed to continuing on unabated?

IOW, does what happens to the bullet after it leaves the gun have any effect on the gun?

3. Feb 20, 2006

### physicsilliterate69

Okay...the gas leaving the rocket has nothing to do with the rocket taking off. There was nothing to push against. Am I correct?

4. Feb 20, 2006

### physicsilliterate69

Okay...wait a minute. I think I have something. Both sides of the equation must equal the same thing. So, rocket propulsion is based on conservation of momentum. If gravity is absent, the downward momentum of the exhaust gases is equal in magnitude and opposite in direction to the upward momentum of the rocket at all times.