# Is weight universal?

1. Nov 24, 2009

### northern expo

not sure if this question has been asked before, but is the weight of an object the same in a vaccum compared to weight in an atmosphere. the concerns of this question is that do we need to consider that our atmosphere provides some boyancy on our mass to change the value of weight.

2. Nov 24, 2009

### Staff: Mentor

If by "weight" you mean the downward force that a stationary object exerts on whatever is supporting it against gravity, yes, it depends on the buoyant force of the surrounding medium. If the medium is air, we usually ignore this effect, especially in introductory textbook examples, but if you need a lot of precision you may have to take it into account. Or if the medium is dense, e.g. water.

3. Nov 24, 2009

### mgb_phys

Weight is the force an object feels due to gravity.
So you only feel it if you are standing on (or near) an object

Yes the boyancy of air has a small effect on the downward force.
I suppose you could split hairs and say this doesn't change the weight, it only changes the total downward force but in everyday language this is the weight - you could always say "apparent weight" if you want to be more precise

4. Nov 24, 2009

### northern expo

what is the bouyant force of air??

5. Nov 24, 2009

### mgb_phys

If a person has a mass of 75kg and the density of water, they will have a volume of 0.075m^3 so the bouyancy of air only makes about 100g difference.

6. Nov 24, 2009

### Staff: Mentor

It gets more complicated when you want to weight something - for really precise work you should take into account density difference between object and weights material.

7. Nov 24, 2009

### gabrielh

Buoyancy is the force exterted upward to keep a given object afloat. In the case of air, buoyancy is proportional to altitude. As an object rises in the air its buoyancy decreases as the density of the surrounding air decreases and vice-versa as the object falls.

Hope I was able to explain that well.