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# Rocket Question watch

1. Hi !

I seem to get the right answer when I round up, and use g=10 instead of 9.81... but I don't think I am quite correct.
I have shown it doesn't lift off immediately since the weight is greater than the upthrust initially.
I know the time taken should be for the weight to balance the upthrust exactly, I found the difference in weight, divided by g and then divided by the 16 rate. but this would take into account the fuel AND rocket? The units work out and kind of so does the answer i got of 1.1 (rounded) but I'm a little confused. could someone please provide a solution to this?
Many thanks
2. (Original post by marinacalder)
...
Can we get some more dimensions on that pic...?
3. (Original post by RDKGames)
Can we get some more dimensions on that pic...?
4. (Original post by marinacalder)
Hi !

I seem to get the right answer when I round up, and use g=10 instead of 9.81... but I don't think I am quite correct.
I have shown it doesn't lift off immediately since the weight is greater than the upthrust initially.
I know the time taken should be for the weight to balance the upthrust exactly, I found the difference in weight, divided by g and then divided by the 16 rate. but this would take into account the fuel AND rocket?
I'm not sure precisely what you're asking. Can you put up your working, please? You need to take into account the mass of the rocket+fuel, but it's only the mass of the fuel that changes, and you know dm/dt for that, so I can't see where the problem would lie.
5. so its ok to use that rate along with the total mass of rocket and fuel? since the rocket mass doesn't change?
6. (Original post by marinacalder)
so its ok to use that rate along with the total mass of rocket and fuel? since the rocket mass doesn't change?
The rate of change of mass is since the mass of the rocket doesn't change. So yes, it's fine to use the given rate of change on its own.
7. (Original post by atsruser)
The rate of change of mass is since the mass of the rocket doesn't change. So yes, it's fine to use the given rate of change on its own.
YAY! many thanks

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