# Weight of planets

#### chetan

[SOLVED] Weight of planets

Does the weight of planets depend upon the speed of the planet and gravity ?

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#### LURCH

Originally posted by chetan
Does the weight of planets depend upon the speed of the planet and gravity ?
Just to make it clear, are you asking what causes a planet to have its weight, what causes things on the planet to have weight, or how we measure the weight of a planet?

#### marcus

Gold Member
Dearly Missed
Originally posted by chetan
Does the weight of planets depend upon the speed of the planet and gravity ?
hello Lurch and chetan, this is a thought-provoking question.
let's assume that chetan is asking if the MASS of a planet depends on the speed the planet is travelling

there are gravitational interactions that abruptly boost the speed of planets----the "slingshot" interaction with Jupiter for example can suddenly increase the speed of a small planet or asteroid to the extent that it leaves the solar system! We are lucky to have Jupiter where it is because it has "cleaned out" the inner solar system of many comet nuclei and stuff that could hit us, exactly by this "slingshot" effect.

So, imagine you are on a planet that suddenly gains a lot of speed. Does gravity get stronger? Interesting. thanks chetan for the idea

#### Labguy

Re: Re: Weight of planets

Originally posted by marcus
hello Lurch and chetan, this is a thought-provoking question.
let's assume that chetan is asking if the MASS of a planet depends on the speed the planet is travelling

there are gravitational interactions that abruptly boost the speed of planets----the "slingshot" interaction with Jupiter for example can suddenly increase the speed of a small planet or asteroid to the extent that it leaves the solar system! We are lucky to have Jupiter where it is because it has "cleaned out" the inner solar system of many comet nuclei and stuff that could hit us, exactly by this "slingshot" effect.

So, imagine you are on a planet that suddenly gains a lot of speed. Does gravity get stronger? Interesting. thanks chetan for the idea
A change in the "speed" of a planet, defined as one in orbit, would only cause it to spiral inward if V is lost, or move outward from the parent body if V is gained. Too much +V could change the orbit, or sling it completely away from an orbit.

The planet's "weight" (mass/density) would not change, but it's potential energy would change in direct proportion to the change in V2. But, if the V approaches what would be called "relativistic velocity", it is a whole new ball game. The only objects I know of at relativistic V's are Neutron stars orbiting each other or orbiting a BH, there may be others (astronomical "bodies"), not meaning particles.

EDIT:
But, Marcus (getting real), you already know that and more.

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#### Phobos

Staff Emeritus
Gold Member
Last edited by a moderator:

#### chetan

Re: Re: Weight of planets

Originally posted by LURCH
Just to make it clear, are you asking what causes a planet to have its weight, what causes things on the planet to have weight, or how we measure the weight of a planet?

How we measure the weight of a planet?

#### Phobos

Staff Emeritus
Gold Member
Click on the link I provided above.

#### russ_watters

Mentor
Re: Re: Re: Weight of planets

Originally posted by chetan
How we measure the weight of a planet?
Maybe we need to get more basic. Weight and mass are not the same thing. Mass is defined as "A property of matter equal to the measure of an object's resistance to changes in either the speed or direction of its motion." Its essentially the quantity of matter in an object. (holding off on realtivity for the time being)

Weight is the gravitational force between two objects. Ie, I weigh 145 pound when placed on the earth and the earth weighs 145 pounds when placed on my feet. On the moon, I weigh about 22 pounds.

So you see, the "weight" of a planet is a concept without really any meaning. Physicists and astronomers only really concern themselves with the MASS of a planet.

So the answer to your question if you mean "how do we measure the mass of a plant?" is that you measure it from watching things orbit it.

Just an aside, I think some of those with more specific knowledge of physics (ie, knowing more than me) tend to look for a complicated answer in situations where it isn't warranted. This appears to me to be a basic question not needing any of the high end complexities that can be applied.

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#### Labguy

Re: Re: Re: Re: Weight of planets

Originally posted by russ_watters
Just an aside, I think some of those with more specific knowledge of physics (ie, knowing more than me) tend to look for a complicated answer in situations where it isn't warranted. This appears to me to be a basic question not needing any of the high end complexities that can be applied. [/B]
Very good point we should all keep in mind.

#### hawaiidude

just so no ones confused, mv^2/r=GmM/r^2.....g is gravity but i thought there was no gravity in space?

#### Labguy

Originally posted by hawaiidude
just so no ones confused, mv^2/r=GmM/r^2.....g is gravity but i thought there was no gravity in space?
Sure there is. Its everywhere, even "deep-space" between galaxies, but there it would be very weak and would be "tugging" you from very many directions, seemingly randon.

Labguy

#### russ_watters

Mentor
Originally posted by hawaiidude
just so no ones confused, mv^2/r=GmM/r^2.....g is gravity but i thought there was no gravity in space?
In the classical sense, space itself doesn't have "gravity" - gravity is a force between two bodies with mass (as the equation says). So that means ANY two bodies with mass ANYWHERE have a calculable gravitational attraction between them.

You are probably thinking about astronauts in orbit. In orbit, they still have a gravitational attraction to earth and its only slightly lower than on the surface (a couple of percent). They just don't feel it because they are in freefall.

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