Why is earth considered to have zero potential potential energy

1. Oct 21, 2009

Rohit Mallya

Why is earth considered to have zero potential[potential energy]...???

2. Oct 21, 2009

Staff: Mentor

Re: hi

You mean gravitational potential energy at the surface of the earth? Convenience.

3. Oct 21, 2009

Rohit Mallya

Re: hi

But why...??

4. Oct 21, 2009

Rohit Mallya

Re: hi

What is the reason Sir...???

5. Oct 21, 2009

Staff: Mentor

Re: hi

As Russ said, it's for convenience in calculations. For example, the gravitational potential energy for an object of mass m lifted a height h is PE = mgh, where g is the acceleration of gravity at the surface of the Eargh, 0.9m/s^2. You will usually not add on a constant offset of potential energy in your calculations, you just call the PE zero somewhere that is convenient in your situation. It may be the surface of the Earth, or the surface of the Moon, or the top of some tower, whatever.

It doesn't matter because you are usually calculating things that have to do with the change of PE, so you can put your zero point anywhere that is convenient, and just deal with the deltas from that zero point.

6. Oct 21, 2009

Rohit Mallya

Re: hi

ok....
But is really P.E. zero...??

7. Oct 21, 2009

Rohit Mallya

Re: hi

Anybody help..!!

8. Oct 21, 2009

Staff: Mentor

Re: hi

PE is relative. In one frame of reference, it can be zero and in another it can be something else.

9. Oct 21, 2009

Rohit Mallya

Re: hi

Please give examples for each frames....

10. Oct 21, 2009

bp_psy

Re: hi

An object has 0 potential energy with respect to the point it is at.
If you mean can we chose another point lower than the ground as 0 than sure. An example would be choosing the bottom of a well as 0 and using mgh to find velocity of an object that was dropped from ground level.

11. Oct 21, 2009

Staff: Mentor

Re: hi

Not possible: there are an infinite number of possible reference frames. Frame choice is completely arbitrary.

12. Oct 22, 2009

nnnm4

Re: hi

Let me try to be more precise. Potential energy is basically a way to express a particular type of force in a more convenient way. However saying that the potential energy at some point is 0 has NO PHYSICAL MEANING AT ALL. The only physical part of potential energy is a difference in it's value between 2 points. So we can arbitrarily define a zero potential energy anywhere we please (where it is convenient is the best choice). Consider if we define the gravitational potential energy at the surface of the earth to be 4 in arbitrary units. The potential at 10 length units above that is simple 4 + mg*10, so the DIFFERENCE is simply mg*10. If we define the potential at the surface to be zero, at height 10 the potential energy is 0 + mg*10, so the difference is still mg*10.

13. Oct 23, 2009

Rohit Mallya

Re: hi

But what if we choose the earth's potential as infinity? then the potential diffeerence won't be same na!?!

14. Oct 23, 2009

arildno

Re: hi

Infinity is not a real number, so you shouldn't choose that one!

15. Oct 23, 2009

Bob_for_short

Re: hi

You know, the Newton equation m*a = F does not contain a potential but a potential difference: F = -delta_U/delta_z. Whatever absolute value of potential is chosen U0 in U(z), it disappears (cancels) in the potential difference. So it is chosen arbitralily.

It is not an internal energy. It is a possibility to make a work when one moves along z. The work is just the potential differnece U(z2) - U(z1) where U0 cancels.

16. Oct 23, 2009

Rohit Mallya

Re: hi

But Sir potential energy means internal energy na?

17. Oct 23, 2009

Rohit Mallya

Re: hi

hidden energy!

18. Oct 23, 2009

Rohit Mallya

Re: hi

Potential energy=Hidden energy!

19. Oct 23, 2009

arildno

Re: hi

No, it does not.

20. Oct 23, 2009

Bob_for_short

Re: hi

Not at all. By definition the potential energy depends on relative positions of interacting bodies. What happens inside each body (tempreature) is not related to this definition.