# The Naked Scientists Forum

### Author Topic: Is gravitational motion perpetual?  (Read 1089 times)

#### jeffreyH

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##### Is gravitational motion perpetual?
« on: 24/10/2015 15:15:43 »
The question is a little misleading. If we have a perfect sphere with a perfectly frictionless surface and set in motion an object along this surface would its inertia be changed by gravity. That is would gravity eventually stop the motion of the object? Take the mass of the sphere as being the same as that of the earth.

#### PmbPhy

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##### Re: Is gravitational motion perpetual?
« Reply #1 on: 24/10/2015 18:23:46 »
Quote from: jeffreyH
The question is a little misleading. If we have a perfect sphere with a perfectly frictionless surface and set in motion an object along this surface would its inertia be changed by gravity.
What do you man by its inertial changing? Inertia refers to the mass of a body so I can't see how a body sliding along the surface of a sphere would change. What made you think it would?

Quote from: jeffreyH
That is would gravity eventually stop the motion of the object? Take the mass of the sphere as being the same as that of the earth.
In general relativity one can show that the system will emit gravitational waves which carry off energy and thus the motion will eventually cease. But that would take an extremely long time to happen. However using Newtonian gravity there is no reason that motion will ever change.

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#### jeffreyH

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##### Re: Is gravitational motion perpetual?
« Reply #2 on: 24/10/2015 19:36:15 »
Since inertia is the resistance to a change in state of motion it has to be affected by gravitation. It is as if the inertia decreases. It is more and more likely to change its state of motion. I know inertia isn't considered in this way but there are reasons I am looking at it like this.

#### Bored chemist

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##### Re: Is gravitational motion perpetual?
« Reply #3 on: 24/10/2015 19:43:11 »
I know inertia isn't considered in this way but there are reasons I am looking at it like this.
OK, so why are you doing what you know everyone else thinks is wrong?

#### jeffreyH

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##### Re: Is gravitational motion perpetual?
« Reply #4 on: 24/10/2015 20:07:08 »
At one point everyone thought there was an absolute space and an absolute time. We now think of mass as relativistic. It increases with velocity. So if it increases then inertia changes.

#### evan_au

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##### Re: Is gravitational motion perpetual?
« Reply #5 on: 24/10/2015 22:20:48 »
Quote from: jeffreyH
If we have a perfect sphere with a perfectly frictionless surface
This only happens in "Physics Land".
In reality, the various types of friction, gravitational distortion of the Earth and interaction with Earth's magnetic field will slow down the object faster than radiation of gravitational waves.

#### jeffreyH

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##### Re: Is gravitational motion perpetual?
« Reply #6 on: 24/10/2015 22:32:08 »
Physics land can lead to new insights.

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##### Re: Is gravitational motion perpetual?
« Reply #6 on: 24/10/2015 22:32:08 »