# Energy necessary to put something in orbit

1. Apr 19, 2017

### undividable

1. The problem statement, all variables and given/known data
Object A is at rest relative to the earth and we want to put it in orbit around the earth, how much energy is necessary to do so?

2. Relevant equations

Ki+Ui=Kf+Uf+energy input

3. The attempt at a solution

what i dont understand is, if the body is at rest, Ki=0, but dont we need a certain inicial velocity to put it in orbit? So shouldn't Ki have a value?

2. Apr 19, 2017

### andrevdh

I think that is exactly what the question requires you to answer. That is how much initial kinetic energy is required to put the rocket in orbit around the earth at a certain distance above the surface of the earth.

3. Apr 19, 2017

### Janus

Staff Emeritus
Point 1: If Ki+Ui is the initial total energy of the object while at rest with respect to the Earth and Kf+Uf is the final energy of the object in orbit, Then why would you add energy to the total final energy to get the total initial energy?

Point 2: Just relating initial energy, added energy, and final energy is not enough, you also need to know how to determine what final energy is needed for the orbiting object.

4. Apr 19, 2017

### haruspex

Is that the whole question? It is very poorly worded.
What does at rest relative to the Earth mean? I could read that as including Earth's rotation, so at the right radius it would be in geostationary orbit already.
It does not say the radius is to be unchanged.
It should provide some variable in terms of which you can express the answer; that could be the height (from Earth's centre), or its GPE, or ...?