# Help with astronomy

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Thread starter 4 years ago
#1
How does the universe expanding forever relate to the space rocket motion?

Thanks in advance!
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4 years ago
#2
I guess you're supposed to realise that the amount of time a rocket can burn it's engines for can't move it far enough for cosmological expansion to make any appreciable difference to the shape of the graph.

fag packet calculations
Hubble constant is ~ 70 km/s/Mpc - so 70 m/s/kpc

nearest star to the solar system is ~1.3x10^-3 kpc distant - this is still a long way further than we're able to send a rocket

typical speed for a man made space object (e.g. ISS) 7.6 km/s

so I'd say C - it'll accelerate at an increasing rate as it uses up it's fuel (which is the main part of a rocket's mass at launch)
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Thread starter 4 years ago
#3
(Original post by Joinedup)
I guess you're supposed to realise that the amount of time a rocket can burn it's engines for can't move it far enough for cosmological expansion to make any appreciable difference to the shape of the graph.

fag packet calculations
Hubble constant is ~ 70 km/s/Mpc - so 70 m/s/kpc

nearest star to the solar system is ~1.3x10^-3 kpc distant - this is still a long way further than we're able to send a rocket

typical speed for a man made space object (e.g. ISS) 7.6 km/s

so I'd say C - it'll accelerate at an increasing rate as it uses up it's fuel (which is the main part of a rocket's mass at launch)
So its weight dreases and upward resultant force increases with time so does its acceleration?

Thanks!
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4 years ago
#4
(Original post by mystreet091234)
So its weight dreases and upward resultant force increases with time so does its acceleration?

Thanks!
that's definitely going to be the dominant effect - >80% of launchpad mass is fuel so for a constant thrust from the engine the astronauts are really going to get pushed back into their seats when the fuel is close to running out.
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