Why is earth considered to have zero potential potential energy

[Rohit Mallya]

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

 

[russ_watters]

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

 

[Rohit Mallya]

But why...??

 

[Rohit Mallya]

What is the reason Sir...?

 

[berkeman]

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.

 

[Rohit Mallya]

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

 

[Rohit Mallya]

Anybody help..!

 

[russ_watters]

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

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

 

[Rohit Mallya]

Please give examples for each frames...

 

[bp_psy]

Anybody help..!

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.

 

[russ_watters]

Please give examples for each frames...

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

 

[nnnm4]

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.

 

[Rohit Mallya]

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

 

[arildno]

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

 

[Bob_for_short]

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

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.

 

[Rohit Mallya]

But Sir potential energy means internal energy na?

 

[Rohit Mallya]

hidden energy!

 

[Rohit Mallya]

Potential energy=Hidden energy!

 

[arildno]

But Sir potential energy means internal energy na?

No, it does not.

 

[Bob_for_short]

But Sir potential energy means internal energy na?

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.

 

[Rohit Mallya]

ok so P.E. of Earth is takes to be 0 for convenience sake correct Sir?

 

[arildno]

ok so P.E. of Earth is takes to be 0 for convenience sake correct Sir?

YES.

It is the potential DIFFERENCE that matters, NOT the potential values themselves.

To understand this, let a, b and c be numbers.

Then, we may set up:

(a+c)-(b+c)=a-b

Do you see that the right-hand side is INDEPENDENT of the number "c"?

Thus, if you at a point A choose to define the potential as either "a" or "a+c" (i.e, DIFFERENT from each other), as long as you at a poiny B then have values "b" or "b+c", respectively, the potential DIFFERENCE will be the same, namely a-b.

Got that?

 

[Rohit Mallya]

yes Sir but if we forget and try to calculate Earth's potential what will be its value?

 

[Bob_for_short]

yes Sir but if we forget and try to calculate Earth's potential what will be its value?

The body's potential on the Earth will anyway depend on from which position we bring a body to the Earth. If from infinity, then it will be G*M_Em/R_E.

 

[Rohit Mallya]

no I'm asking for earth.what will be Earth's actual value of potential energy?

 

[arildno]

no I'm asking for earth.what will be Earth's actual value of potential energy?

It doesn't HAVE an "actual", true value.

What is real are the potential differences between the Earth and other objects.

Your question is like:

What is the ACTUAL place of the origin on a road, from which I measure distances in either direction?

Do you understand that that is a meaningless question?

 

[Rohit Mallya]

ok Sir thanks a lot.

 

[Rohit Mallya]

It is very much relative na Sir?

 

[arildno]

It is very much relative na Sir?

Correct. Only the difference in potential values matter, not the potential values themselves.

 

[Bob_for_short]

Potential is a potential capability to make work. If potential does not change, no work is done due to it. In this case it is not hidden but unemployed. So the absolute value does not say anything. Only differences does: U₂ - U₁ = work to change the bodies relative distance from the original position-1 to the final-2. Think of stretching and compressing a spring.