My own body and its mass

1. Dec 18, 2008

chis

A value for gravity

Can the equation E=MC2 give you a value for a bodies gravity? If so whats the procces for working out mass?

Thanks folks
Chris

Last edited by a moderator: Dec 18, 2008
2. Dec 18, 2008

chis

Could I workout my own mass void of any gravity and in a vacum?

3. Dec 18, 2008

Staff: Mentor

Could you start out holding a bowling ball of known mass?

4. Dec 18, 2008

Staff: Mentor

Sure, why not? (Since your mass does not depend on those things, it would be the same as it is now.)

[Perhaps I don't understand the point of your question.]

5. Dec 18, 2008

chis

Eh! no comprendy. Sorry.

6. Dec 18, 2008

chis

If our weight is the basis of determining our mass, is it not gravity that determines our recordable weight. How do you remove gravity from the result and get your undiluted mass and energy value?

7. Dec 18, 2008

Staff: Mentor

You can use a weight measurement to determine your mass, but your mass does not depend on gravity being present. If you go into outer space, your mass doesn't change.
What do you mean by "undiluted mass and energy"?

8. Dec 18, 2008

chis

I though that gravity has the ability to increase velocity and velocity increases mass so being stationary in empty gravityless space would be pure a reading of personal mass.

9. Dec 18, 2008

Staff: Mentor

Well, if you jump off a cliff (on Earth) gravity will increase your speed, all right. If a body is moving with respect to you, you will measure its so-called relativistic mass as increasing, but that's better described as an increase in kinetic energy. (It does get harder to accelerate something that's moving fast.) Your intrinsic mass ("rest" mass) doesn't change.

10. Dec 18, 2008

chis

If we are constantly moving, gravity around the sun and bombing through an expanding cosmos, is kentic energy (mass) not a vasty variable polutant, or are there assumptions to factor it out? Or do we just take our earthly mass as read?

11. Dec 18, 2008

Naty1

Re: A value for gravity

No, mass is independent of gravity;
Weight of a mass is related to gravity via F=Ma=Mg=W...
your own mass, for example, remains constant on earth or our moon, but your weight varies according the gravity....

Ssssseeeeeeeeehhhh...go away for two minutes and all sorts of posts pop up...competitive group!!!!! Can't I even take a break to play with the dog???

Last edited: Dec 18, 2008
12. Dec 18, 2008

chis

What was that equation Naty? Can you expand it a wee bit?

13. Dec 18, 2008

HallsofIvy

Staff Emeritus
Do you mean "F= Ma= Mg= W"?

F= Ma is "force equals mass times acceleration". On the surface of the earth, g is specifically the acceleration du to gravity, so that force is Mg which is what we mean by "weight".

It is also true, for example, that if you attach an object to a spring, the period of the motion will depend on the mass. That can be used to determine the mass of an object aboard a space station or shuttle in orbit- or if you were in interstellar space far away from any massive body so there would be no measurable gravitational pull.

14. Dec 18, 2008

chis

Chris

15. Dec 18, 2008

DaleSwanson

I think I can clarify something for you. The Theory of Relativity is named that because everything is relative. What this means, is that if you were motionless in space and you saw a comet go by, it would be just as true to say that the comet was not moving and you went by it. Your speed must always be defined as relative to something else. Here on Earth we usually define speeds as relative to the Earth, and we define the Earth's speed as it orbits the sun in relative to the sun.

It's a hard concept to accept at first, but there is no "true" or "real" answer to which object is moving, it's all relative to where you are measuring from. For example if we had a small planet, and a spaceship out in interstellar space and we launched from the planet and moved forward at 99% of the speed of light people on the planet would observe our mass as increasing, and if they could see clocks on our ship they would see them moving slower. However, we would measure the exact same effect on the planet, as if we had remained still and they had accelerated to .99c (because both answer are equally true). If there were an ray of light passing by both us and the planet (and anyone else) would measure the speed as exactly the same. Even as we were moving at 99% the speed of light if we turned on our headlights the light would pull away from us at the exact same speed as if we were still (because again relative to ourselves we are).

Now in the above example you could still measure relative to the other stars and see that the planet was not moving relative to them, and you were, thus it would be tempting to say you were really moving they the planet really was still. If you took all the other stars away, and the planet how would you know you were moving, you wouldn't, because relative to your only reference point (yourself) you are not moving. There would be no way for you to detect the increase in mass from moving because all your instruments would have gained mass as well. After many generations on our spaceship, after all who knew the setup for the experiment were dead, if a comet passed by would we say we moved past it, or it flew by us? Both would be equally true, it would just depend on your point of reference.

Rest mass is the mass of an object while no accounting for any gains due to Relativity. You may think you could detect the gains when you began to move, but you would not, at least not if you and your instruments we part of the mass.

I hope I explained this well, I'm not an expert, but I had the same thoughts as you before (that you could tell if an object was moving by detecting gains in mass, and thus could say for sure that an object was truly motionless if it's mass = it's rest mass).

Last edited: Dec 18, 2008
16. Dec 18, 2008

Gnosis

Re: A value for gravity

Naty1, we're physicists, so we don't "play" with dogs rather, we analyze their progress after attempting to teach them the physics associated with trajectories, by throwing them a stick.