basic physics question

[bish]

Don't know why I was thinking about this - must have had a quiet moment.  From what I remember at school, if you lift something off the ground it has potential energy. If the Earth suddenly vanishes does it still have the potential energy?

[Gutross]

I am no Physics Major, but with out a Gravitational body i would think not.  makes you think though

[JamesF]

I'd say no.  The potential to gain energy comes because of the Earth's gravity.  Remove that and the potential energy disappears too.

Though you may have potential energy as a result of the Moon's gravity instead.

James

[bish]

Thanks James.

[Dom1961]

I would guess, that as it is lifted, the object gains gravitational potential energy. If you take away the gravitational body(in this case earth which is moving) the gravitational potential energy, with nothing holding the object back, will be transferred to kinetic energy as it flies off into space. You could think about this in terms of the use of "sling shot" which space craft use to gain momentum. As it goes round a gravitational body, kinetic energy transfers to gpe, but as it leaves the body, the craft gains kinetic energy as it loses gpe from leaving the body. As the further out you get from a gravitational body, the less effect gravity has on the object. So although I have gone quite far off topic, I think it will get transferred to kinetic energy. But if we were talking about a static gravitational body, I have absolutely no idea. I'll let someone who's actually out of secondary school answer that

[Calculon]

No it does not. Quite how you would manage to get the Earth to instantly vanish is another matter!

[bish]

No it does not. Quite how you would manage to get the Earth to instantly vanish is another matter!

Yes, that is the hard part if you conduct my experiment!

[JamesF]

David Blaine isn't busy at the moment, is he?

James

[Ruud]

Assuming that the Earth and the object are the only two things in the universe: the energy of the object would increase to zero as the Earth disappeared.

Before and after  the Earth disappears the kinetic energy of the object is zero. (We assume it to begin and stay at rest.)

As long as the Earth is still there, the object does not have its maximal potential energy (it would have its maximal potential energy if it were at infinity, but it is not)

After the Earth has vanished, the object remains at rest (kinetic energy stays zero)  and has the equivalent of its maximum potential energy with respect to the Earth.

Yet, there no longer is an Earth, and the objects potential energy with respect to a non-existent Earth is zero.

----

In classical physics, objects have zero potential energy at infinity. When they start to drop into a gravity field, their potential energy decreases and their kinetic energy increases by the same amount.

[ronin]

As the potential energy for a near earth object the potential energy is P = mgh.

As the earth is disappeared then g = 0 suddenly, so potential becomes 0 through either g = 0 or I suppose h = 0 as well.

[Ruud]

Epot = mgh is an approximation that works fine close to the Earth, but not if you move farther into space. If it were to work at any distance, the escape velocity of Earth would be infinite.

Here is an account of what's really going on in classical physics.  Epot = -G * M_Earth * M_object / distance  is the correct formula to user.

[ollypenrice]

How does physics respond to that which is physically impossible?

Olly

[Ruud]

That's easy, Olly. You just compare the before and the after, and ignore the in-between.

[Tiki]

How does physics respond to that which is physically impossible?

Olly

Just the same as the physically possible. If it didn't then it wouldn't be physics.

OP: When you remove the source of the field (earth) everywhere has the same potential.  It is the same as if you were to hold a nail close to an electromagnet and then turn off the electric. No field no pull on the nail.

[Alien 13]

Wouldnt the sudden removal of the earth cause the the space/time distortion that had been created to spring back flinging the object away.

Alan

[Ruud]

Wouldnt the sudden removal of the earth cause the the space/time distortion that had been created to spring back flinging the object away.

Alan

Wonderful, Alan, I think you have just invented the space trampoline.

[Alien 13]

Wonderful, Alan, I think you have just invented the space trampoline.

I was thinking about a similar experiment with the Sun and Earth, now if the Sun vanished we would not know for 8 mins but would the gravity "bounce" effect us sooner.

Alan

[JamesF]

I was thinking about a similar experiment with the Sun and Earth, now if the Sun vanished we would not know for 8 mins but would the gravity "bounce" effect us sooner.

Alan

If it did, wouldn't that mean that something had to travel faster than the speed of light?

James

[Dom1961]

No, it will be nearly instantly? Because the gravity from the sun isn't there, the space time around it will not be warped. Therefore time for us will run faster so it will seem almost instantly?

[bish]

Thanks for all of your interesting replies. I thought I might just get one 'yes' or 'no'.

[Dom1961]

Physics never has a yes or no answer:D