# Energy Conservation [closed]

Supose that I throw a ball on the surface of the Earth at a given instant t. Chosing the referencial as the surface of the Earth, the PE is zero (and the KE is different from zero). Imagine that when all the KE is converted to PE (when the ball stops in the higher point of the trajectory) the Earth instantly disapears. Let's assume the Earth can disappear in two ways:

Case 1: The Earth is removed from the universe;

Case 2: The Earth is removed from the Solar System and instantly appears in other place in the universe, far away from the Solar System;

Well, in Case 1 the the universe itself wouldn't be a isolated system, so the laws won't work (is that right?)

So let's think in Case 2: The ball wouldn't be afected from the gravitational pull of the Earth anymore so the PE that depends on earth's gravity would be lost, and I really mean lost, and that doesn't make sense. Then I remembered the special relativity theory that tells us when the earth disappears, a wave from the space-time fabric is generated and travels at the speed of light as a signal, because the Earth was creating a high depression on the space-time fabric before was removed. Can we interpret the wave as a form of energy converted from the delocalization of the Earth's mass? So it's correct to imagine that the wave "hits" the static ball and that ball would adquire again velocity, that we can translate to a KE? But wouldn't that KE be way bigger than the PE that the ball had on Earth? How conservation of energy works in this case? Thank you for your time

Note: For case2 instead of the Earth disappearing instantly imagine a perfectly elastic collision between Earth (suposing Earth is like a giant billiard ball) and another planet (as a giant billiard ball too) at the instant the little ball thrown in the air stops. That collision is so fast that the Earth and the other planet instantly move at oposite directions before our little ball starts to "fall". Would the ball "fall" without any of the planets being around? –

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## closed as off-topic by BMS, Brandon Enright, John Rennie, DavePhD, Kyle KanosMay 14 '14 at 13:01

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You question about energy conservation of the simple little useless ball and you don't care about the disappearance of the mass energy of the whole Earth? :-O I would say you may not speak about energy conservation if you let the Earth disappear. – Wizzerad May 13 '14 at 22:44
You're right, but think in this way: Imagine a perfectly elastic collision between Earth (suposing Earth is like a giant billiard ball) and another planet (as a giant billiard ball too) at the instant the useless ball thrown in the air stops. That collision is so fast that the Earth and the other planet instantly move at oposite directions before our useless ball starts to "fall". Would the ball fall withou tany of the planets being around? – 21Brunoh May 13 '14 at 22:54
You are forgetting that the ball's gravity is pulling from the Earth, and so the Earth will not go so far for the same kick. There is your "missing" energy. – Davidmh May 13 '14 at 23:00
So you're saying it's impossible to change Earth's position almost "instantly" making the little ball out of Earth's (gravity) range ?Is that because of the mass of the Planet and the fact objects that travell at high speeds (almost at c, speed of light) are almost massless? – 21Brunoh May 13 '14 at 23:14
No offense, but these common questions that involve the sudden vanishing of the Sun or Earth, etc. are meaningless. It is like saying "Assume I have a ball of Flubber". Well, Flubber is more feasible (Black and White Flubber). If you have a mechanism that can make this happen, then OK. If not, it is like assuming you have little guys who can yell at each other whenever an electron goes through the slit they are watching without affecting the interference pattern. Given that, you are asking a simple Newton's Law question. With gravity gone, object at rest will..... – C. Towne Springer May 13 '14 at 23:43