Viewing a single comment thread. View all comments

Alkaven t1_j2xnbi6 wrote

My understanding of orbits is that if every particle in the universe vanished except the Earth and the Sun, the Earth would continue in its orbit, because it is in free-fall around the Sun.

My understanding of conservation of energy makes me think that this is impossible. I was told in HS physics that the Earth is constantly accelerating--not because it gets faster and faster, but because it's changing directions by going in a circle ellipse instead of a straight line. F=ma, so it takes force to change the direction. That force is provided by gravity... but here I get confused. Can gravity just provide this force forever? Does that not mean that it's generating energy (from nowhere)?

I'm ten years out of high school so I can't just raise my hand and ask the teacher :( I suspect that I knew the answer to my question at some point and have since forgotten it.

My Google searches somehow turn up all sorts of sources debunking flat earth theory, which is not what I'm trying to do here lol.

1

Weed_O_Whirler t1_j2xrwxj wrote

It doesn't take energy to apply a force. Think of a book sitting on a table. Gravity is pulling the book down (aka, a force pulling down) and the table is pushing up on the book (aka, a force pushing up). And it can just sit there forever. Obviously the table doesn't need energy to do that.

No different than the Sun keeping the Earth in orbit. The energy of the Earth/Sun system stays constant (assuming nothing else in the universe). The force the Sun is providing on the Earth isn't changing its kinetic energy (since kinetic energy only depends on the speed of the object, not its velocity), so conservation of energy doesn't get violated here.

4

Alkaven t1_j2xxtr4 wrote

Wow okay, thanks! I was totally confounding force and energy. The book example is really helpful.

1