Perpetual Motion In The Gravitational Field

physio

I have resnick and halliday but I don't quite like that book. I wanted to know the basics of force, momentum, energy etc. By reading R.Feynman I actually "UNDERSTOOD" what force really is. I understood why ƩF=ma, how momentum is closely related to the concept of inertia and how Newton has not done much but has taken other peoples results and integrated them into one whole. But above all I have understood there are no really's in science, only models (really!). :) Resnick and halliday doesn't quite give me that kind of an insight, it doesn't engender a joy of learning physics. But if you can suggest some other books which explains the basics really well than that would be wonderful. Thanks!

Vorde

If Feynman is working for you that's great, I only shared my experience in case you were in a similar boat as me. Best of luck to you!

physio

Vorde, your explanation really helped clear a lot of stuff...Thanks again!

P.S. Which books have you used to advance your physics knowledge.??

Lsos

Don't know if this is too little and/ or too late, but here's my .02:

OP, even IF the gravitational force from the earth was 0, that doesn't mean there is no more potetnial energy. The energy is still there, and you can easily get it back by simply pushing the object back towards the earth's gravitational well.

E=mgh is an approximation assuming an even gravitational field. As you know, for larger distances g will change, and therefore we have to modify the formula to take this into account (I'm not good at calculus, but I imagine we need to integrate). Basically we solve many E=mgh equations for all the different values of g, and we add them all together. This means that as you get further away, the gain in potential energy might diminish, but the potential energy will ALWAYS go up. Even IF at some large distance, mgh=0, this value is simply added on top of all the other values. Those other values never go away.

As for your off-topic question of why the universe is expanding despite gravity attracting everything...that's a VERY good question. It's a very good question becaue indeed it doesn't make sense, and many people are scrambling trying to figure out the answer.

physio

Thanks Lsos I understood the answer from yours and Vorde's explanation, that even if the object is very far away there will be some potential energy because F=Gmm/r^2 and F=0 only when the distance is infinite...Thanks!!!!