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 Post subject: rocket eqnPosted: Sun, 22 Apr 2012 18:17:30 UTC
 S.O.S. Oldtimer

Joined: Sat, 21 Jan 2012 03:59:22 UTC
Posts: 182
sorry trying to delete

Last edited by DgrayMan on Sun, 22 Apr 2012 18:25:02 UTC, edited 1 time in total.

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 Post subject: Re: Check Velocity equationPosted: Sun, 22 Apr 2012 18:21:50 UTC
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Joined: Mon, 29 Dec 2008 17:49:32 UTC
Posts: 6071
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DgrayMan wrote:
I am trying to find initial velocity to reach a certain height

So here's what i got

v^2 = 2g((R^2)/(R+h))-2gR+V(0)^2

v0^2 = 2gR - 2g((R^2)/(R+h)) + v^2

so v^2 is 0 cause at the velocity of h the rocket stops. and now solve for V0

v0 = √(2gR) - √(2g((R^2)/(R+h)))

R= 6378.1 km
h= 450 km
g= 9.80665 m/s2 = 127094.184 km/hr2

v0 = √((2)(127094.184)(6378.1)) - √(2(127094.184)((6378.1^2)/(6378.1+450)))

v0 = 1328.1287 km/hr = 368.9246 m/s

Did i solve this correctly?

when i try to do this in metres instead of kiometers using the variables
R= 6378100 m
h= 450000 m
g= 9.80665 m/s2

i get v0 = 374.7107 m/s

is this more or less accurate? are my units of measurement correct when i plug them in the equation?

No!

does not mean

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 Post subject: Re: rocket eqnPosted: Sun, 22 Apr 2012 18:26:09 UTC
 S.O.S. Oldtimer

Joined: Sat, 21 Jan 2012 03:59:22 UTC
Posts: 182
you are right!

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 Post subject: Re: Check Velocity equationPosted: Sun, 22 Apr 2012 18:33:45 UTC
 Moderator

Joined: Wed, 30 Mar 2005 04:25:14 UTC
Posts: 12172
Location: Austin, TX
outermeasure wrote:
DgrayMan wrote:
I am trying to find initial velocity to reach a certain height

So here's what i got

v^2 = 2g((R^2)/(R+h))-2gR+V(0)^2

v0^2 = 2gR - 2g((R^2)/(R+h)) + v^2

so v^2 is 0 cause at the velocity of h the rocket stops. and now solve for V0

v0 = √(2gR) - √(2g((R^2)/(R+h)))

R= 6378.1 km
h= 450 km
g= 9.80665 m/s2 = 127094.184 km/hr2

v0 = √((2)(127094.184)(6378.1)) - √(2(127094.184)((6378.1^2)/(6378.1+450)))

v0 = 1328.1287 km/hr = 368.9246 m/s

Did i solve this correctly?

when i try to do this in metres instead of kiometers using the variables
R= 6378100 m
h= 450000 m
g= 9.80665 m/s2

i get v0 = 374.7107 m/s

is this more or less accurate? are my units of measurement correct when i plug them in the equation?

No!

does not mean

But then that might mean that ....what will we tell the freshmen?

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