Gravity and energy

1. Apr 19, 2012

BobiG

I have a quick question, if an object is moving straight towards say Earth, isn't the gravitational field of Earth going to accelerate that object giving it extra energy? Where does that energy come from? Similar question, you have a hydrogen cloud in space, isn't the mass(energy) of that cloud less than the mass(energy) of that same cloud when it collapses into a star due to gravity? Again where does the extra energy come from? It may be a stupid question but I have to ask!

2. Apr 19, 2012

A.T.

3. Apr 19, 2012

HallsofIvy

If an object "is moving straight towards say Earth", it must have been above the earth and so has potential energy. That potential energy converts to kinetic energy.
There is no net change in energy.

Similar answer- the hydrogen cloud has potential energy due to the fact the hydrogen atoms are separate. There is no net change in "mass-energy".

4. Apr 19, 2012

Khashishi

Not a stupid question at all. I suppose everyone just goes back to Newtonian gravity to answer that one, since I don't think General Relativity has an answer to it. GR says that the object isn't gaining energy when viewed from the appropriate free-falling frame. I think that's a cop-out, because in that frame, the ground is accelerating toward you. Where does *that* energy come from?