# Homework Help: Orbits and Energy

1. Nov 21, 2009

### r_swayze

You launch an engine-less space capsule from the surface of the Earth and it travels into space until it experiences essentially zero gravitational force from the Earth. The initial speed of the capsule is 18,500 m/s. What is its final speed? Assume no significant gravitational influence from other solar system bodies. The Earth's mass is 5.97×10^24 kg, and its radius is 6.38×10^6 m.

I have no idea where to start on this problem. Any help?

2. Nov 21, 2009

### tiny-tim

Hi r_swayze!

Conservation of energy.

3. Nov 21, 2009

### r_swayze

but doesn't the total energy increase as the radius increases?

4. Nov 21, 2009

### tiny-tim

No, total energy remains constant …

why would it not do so?​

5. Nov 21, 2009

### r_swayze

the textbook says it does:

"The total energy of a satellite increases with the radius (in the case of circular orbits) or the semimajor axis (in the case of elliptical orbits). Moving a satellite into a larger orbit requires energy; the source of that energy for a satellite might be the chemical energy present in its rocket fuel."

And dont I need the mass of the satellite to use the energy equations? mass is not given here.

6. Nov 21, 2009

### tiny-tim

ah … they're talking about the total energy for an orbit.

It stays constant throughout the orbit, but of course is different for different orbits.

Although it only talks about circular and elliptical orbits, the same applies to hyperbolic ones (though of course as a matter of English rather than physics, we would call them trajectories rather than orbits ).

In this question, the capsule is following a single orbit (hyperbolic trajectory), and its total energy stays constant throughout.

(incidentally, even a falling object is following an orbit … one that is so elliptical it's just a straight line that goes back and forward though the centre of the Earth )
No, just call the mass m … you'll find it cancels out in the end.

7. Nov 21, 2009

### r_swayze

ok, but then how would I find the radius needed for that orbit? I cant just plug in 0 for F = GMm / r^2, right?

8. Nov 21, 2009