Bypass time of satelite arround Mars

Tags:
1. Jun 7, 2013

antoman

1. The problem statement, all variables and given/known data
On high 150km above the surface of Mars, there is satelite That is ciculating arround Mars. What's his bypass time? Radius of Mars is 3400km.

2. Relevant equations

3. The attempt at a solution

I found equation like this :

t= (2*r(R+h)^(3/2))/(sqrt(g0*R^2))
R=r+h
r=3400km
h=150km

But when i try to calculate like this, i totally miss the actual time. I know bypass time(solution) is 110 minutes.

2. Jun 7, 2013

SteamKing

Staff Emeritus
What value of g0 are you using? What units for r and R are required?

3. Jun 7, 2013

tiny-tim

welcome to pf!

hi antoman! welcome to pf!
let's put that into english
At a height of 150km above the surface of Mars, there is a satellite that is orbiting Mars. What's its period? The radius of Mars is 3400km.​

hmm … don't you need to know either the mass of Mars, or the gravitational acceleration (g) at the surface?

4. Jun 7, 2013

antoman

g0=9,81m/s^2

hmm … don't you need to know either the mass of Mars, or the gravitational acceleration (g) at the surface?

i could calculate g at surface using g=g0*R^(2)/(R+h)^2, but then again i have no idea what to use it for
.. nevermind g0 is different on mars then on earth so..

5. Jun 7, 2013

antoman

Ok i made some changes, so now

t= (2*r(R+h)^(3/2))/(sqrt(g0*R^2))
r=R+h
R=3400km
h=150km
g0=3,7 m/s^2... assuming i should of know g0 from mars.

I calculated and it came out t=7261412,315 km/s...

6. Jun 7, 2013

SteamKing

Staff Emeritus
You've got R and h in km and g0 in m/s^2. Don't you think you have a problem with your units?

7. Jun 7, 2013

antoman

I used 3.6*10^-3 km/s^2 for g0 when i calculated, so its (km*km^(3/2))/sqrt(km*s^(-2)*km^2). Then km^(3/2)/km^(3/2)=1, so all it stays is km*s.
Repair me if im wrong :)

8. Jun 7, 2013

SteamKing

Staff Emeritus
I'm repairing you.
Do you have the right formula?
What if R and r have to be in meters instead of km?

9. Jun 8, 2013

antoman

No, i dont think the formula is right, that's why i asked for help.

10. Jun 8, 2013

SteamKing

Staff Emeritus
11. Jun 8, 2013

antoman

Now i know what was the problem :D

Correct formula is:
t=(2*(...))/sqrt(....)
+ g0 to km/s^2, and it all works out perfect, iven makes more sense :)