# Homework Help: Energy needed to orbit

1. Nov 15, 2008

### crr2010

1. The problem statement, all variables and given/known data

A satellite of mass 200 kg is launched from a site on Earth's equator into an orbit at 205 km above the surface of Earth. What is the minimum energy necessary to place the satellite in orbit, assuming no air friction? I also figured the orbital period to be 5300 s and the orbital velocity to be 7788 m/s

2. Relevant equations
P = -GMm/r
K = .5*m*v^2

3. The attempt at a solution
I plugged the numbers into the kinetic energy equation
.5*200 kg*(7788 m/s)^2
and got 6.07 E 9
1. The problem statement, all variables and given/known data

2. Relevant equations

3. The attempt at a solution

2. Nov 15, 2008

### Janus

Staff Emeritus
What's the potential energy of the satellite in orbit?
What's the total energy of the satellite in orbit?
What's the kinetic and potential energy of the satellite sitting at the equator?

3. Nov 15, 2008

### crr2010

final kinetic = 6.065 E 9 J
final potential = 1.211 E 10 J
final total energy = -6.049 E9 J

initial kinetic = 2.153 E 7 J (using Earth's rotational period of 463.9 s to find earth's v)
now here's where I am not sure
for initial potential energy do you use the total radius or just the radius of the earth?

4. Nov 15, 2008

### Janus

Staff Emeritus
Since it is sitting on the surface of the Earth, doesn't it make sense to use the Earth's radius?