# A quick physics question. **URL:** \ **Category:** Factual Questions\ **Created:** [January 19, 2009, 11:10am UTC](https://boards.straightdope.com/t/a-quick-physics-question/481974 "2009-01-19T11:10:10Z")\ **Posts on this page:** 4\ **Page:** 2
**Author:** ![Mama\_Zappa](https://avatars.discourse-cdn.com/v4/letter/m/71e660/32.png) [@Mama\_Zappa](https://boards.straightdope.com/u/Mama_Zappa)\ **Post date:** [January 19, 2009, 6:18pm UTC](https://boards.straightdope.com/t/a-quick-physics-question/481974/21 "2009-01-19T18:18:00Z")
> [@YamatoTwinkie](#): > > There’s still a mass transfer from the rocket engine into the ball, right? Shouldn’t there be associated momentum with this? Granted, the velocity isn’t going to be very high… But it’s all a single system, as the rocket is attached to / entirely inside the glass ball, So there’s nothing to move away from. The mass transfer would affect the ball’s center of gravity, presumably… hence the hamster wheel analogy. I guess the change in center of gravity causes the ball to fall (and therefore roll). My assumption is that the hamster - er, rocket - wouldn’t move if it were in space and had no gravity for the “falling” to occur. Closest real-life thing I can think of was a Bumble Ball - a ball with blunt prongs all over it. If you picked it up, it would shake in your hands. If you put it down, it would jog along the floor in all sorts of directions. IIRC it was based on the spinning of a motor inside of it. My kids loved it, once they got over being terrified of it! Of course, now I want a rocket-powered hamster ball. That’d be COOOL! 🆒 ---
**Author:** ![YamatoTwinkie](https://avatars.discourse-cdn.com/v4/letter/y/cc9497/32.png) [@YamatoTwinkie](https://boards.straightdope.com/u/YamatoTwinkie)\ **Post date:** [January 19, 2009, 8:23pm UTC](https://boards.straightdope.com/t/a-quick-physics-question/481974/22 "2009-01-19T20:23:12Z")
I think a good analogy is sitting in a canoe, and then moving from the front of the canoe to the back. The bow of the canoe moves forward \*during \* the cg transfer, and then stops when the cg transfer is complete. Our rocket engine fired inside a ball will do the same. You could have accomplished the same thing by just shifting the weight of the rocket with an electric motor, however. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [January 19, 2009, 8:29pm UTC](https://boards.straightdope.com/t/a-quick-physics-question/481974/23 "2009-01-19T20:29:23Z")
> [@](#): > > So, a jet engine cannot work in a vacuum, but a rocket can, as evidenced by all the ones we have sent up. When the rocket leaves our atmosphere, can you still see the booster firing in space, or have they dropped off by then? Every spacecraft going into orbit must necessarily have some sort of rocket burn above the atmosphere. At the moment you leave the atmosphere, you’re either on an escape path (highly impractical and hence highly unlikely), or you’re on a closed orbit which intersects the atmosphere. But a closed orbit which intersects the atmosphere once will intersect it again, which you don’t want to happen. So you need to change which orbit you’re in, which means firing a rocket. ---
**Author:** ![Mama\_Zappa](https://avatars.discourse-cdn.com/v4/letter/m/71e660/32.png) [@Mama\_Zappa](https://boards.straightdope.com/u/Mama_Zappa)\ **Post date:** [January 19, 2009, 10:10pm UTC](https://boards.straightdope.com/t/a-quick-physics-question/481974/24 "2009-01-19T22:10:00Z")
I posed this question to Typo Knig, who is a lapsed physicist (got a PhD, then changed careers). He thought it was intriguing, but what he really wonders is what it would do if it were on a treadmill. /d&r 😃 [Previous page](https://boards.straightdope.com/t/a-quick-physics-question/481974.md?page=1)