# Thread: relativity of weight

1. I have just been watching a video by NASA about weight and mass in low earth orbit.

Mass remains the same.
Weight changes.

This has me thinking. The weight of something is relative to and subsequently defined by the nearest largest body. So on the moon I weigh less than on earth.

By extending this logic does it work the same thoughout the universe - what I mean by this is our moon is defined by the earth, which is defined by the sun, our sun being defined by the nearest sun etc..................

Is this the case? I have asked this before somewhere but can't find it - if I am not mistaken this has to do with special relativity.

Much obliged for any help.

2.

3. Originally Posted by fatman57
I have just been watching a video by NASA about weight and mass in low earth orbit.

Mass remains the same.
Weight changes.

This has me thinking. The weight of something is relative to and subsequently defined by the nearest largest body. So on the moon I weigh less than on earth.

By extending this logic does it work the same thoughout the universe - what I mean by this is our moon is defined by the earth, which is defined by the sun, our sun being defined by the nearest sun etc..................

Is this the case? I have asked this before somewhere but can't find it - if I am not mistaken this has to do with special relativity.

Much obliged for any help.
You have it right.

Mass is the amount of "stuff' that is present. It does depend on any nearby body, but is an intrinsic property of matter in a given inertial reference frame.

Weight is the force that is exerted on mass by a nearby large object. It is proportional to mass, and normally gravity is used in the measurement of mass, either on a balance that compares the gravitational attraction of a known mass with an unknown mass or one a scale by means of the deformation of a spring, of known spring constant, by the force of gravity.

Mass is really the fundamental quantity. Weight is important only in a local context near a large body.

It is not that weight of the sun is defined by nearby large astronomical bodies, there are not. It is that weight of the sun is not a useful context, while mass is very useful.

4. awesome thanks.

I can see that weight of the sun is not a useful context, but it would still work like we would measure the difference between our weight on the earth and the moon and that was the point of my question.

It makes an interesting universe if my weight on earth would be different if earth was put next to a sun with greater mass/gravity, likewise with the sun and our earth in its current orbit.

5. Originally Posted by fatman57
awesome thanks.

I can see that weight of the sun is not a useful context, but it would still work like we would measure the difference between our weight on the earth and the moon and that was the point of my question.

It makes an interesting universe if my weight on earth would be different if earth was put next to a sun with greater mass/gravity, likewise with the sun and our earth in its current orbit.
Normally one only talks of weight in the conntext of a some relatively small object near, essentially on, a much larger body. Otherwise one talks of mass.

It does not make sense to talk about the weight of the sun, but only of its mass. There is no conceivable situation in which one would think of the sun as being essentially on the surface of some larger body. There aren't any larger bodies anywhere close.

The mass of the sun is important because it is responsible for the gravity tha holds the planets in place. The mass of the sun is known pretty well. -- kg.

From that mass you could calculate its weight near a large body of whatever mass strikes your fancy, but it would be a purely hypothetical calculation of little practical import.

Weight is not an intrinsic propert of an object. Mass is. Your weight can be anything, literally anything from zero on up, depending on the mass of nearby objects.

6. thanks - moving on - is dark matter being proposed to have an influence here at all - in this sense I mean on our sun relative to its nearest suns/larger bodies?

7. Originally Posted by fatman57
thanks - moving on - is dark matter being proposed to have an influence here at all - in this sense I mean on our sun relative to its nearest suns/larger bodies?
Dark matter is basically a name given to whatever is responsible for the observed rotatinal rates in many galaxies.

It is observed that the rotation rates of stars in many galaxies are not consistent with the gravity that would be provided by the observed (non dark) matter. Nobody knows why. As a means for explaining what is observed dark matter is hypothesized to exist. There is other evidence for the existence of dark matter in the form of gravitational lensing.

Supposedly dark matter has gravity and perhaps feels the weak force. But it does not interact electromagnetically, hence is dark. Nobody knows what it is, or even for sure if it exists at all.

If if does exist, it is present in our galaxy and interacts gravitationally with all of the stars in the galaxy, including the sun. The local effect would be very small, but the effect on a scale as large as the galaxy would be appreciable.

This question does not have much to do with weight or the OP. What are you studying and what is the real question that is driving all of this ?

8. It did have to do with weight and the OP (in my mind at least!). I was just looking for further explination and possible links to the whole gravity and weight thing.

Its for my perosnal study really, always had an interest in physics.

9. Originally Posted by fatman57
It did have to do with weight and the OP (in my mind at least!). I was just looking for further explination and possible links to the whole gravity and weight thing.

Its for my perosnal study really, always had an interest in physics.
Basically, physicists don't care much about weight. They worry about mass.

Engineers worry about weight, and mass.

Weight gets a lot of press primarily because in English units we measure pounds, and pounds are a unit of force, hence weight. In Metric units the usual measure is in kilograms, a unit of mass. Since all of these considerations are in taken in the context of our earthly environment, there is a direct correspondance between mass and weight, and the two are often confused and used interchangeably in the popular press.

Weight is important on earth and for missoins to the moon and other planets. Otherwise you can safely ignore weight and concentrate on mass.

But if you go on a diet you are really interested in losing mass.

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