# Question about the Archimedes principle and gravity.

1. Oct 11, 2012

### DrSmersh

Hello, I was taking a shower and started to think about balloons and helium and this question came to me:
Wouldn't the things on earth be lighter than say, a similar planet (same gravity) but with no atmosphere? (because of the push from air)
I googled it but im not sure of using the correct keywords.
PS: Im starting engineering and my physics knowledge is not very superior to an AP level (I don't really know how to compare our educational systems sorry :P)

2. Oct 11, 2012

### Staff: Mentor

That depends on exactly what you mean by weight. They will have the same mass, and experience the exact same amount of force downward, but without a fluid surrounding them the effect of buoyancy will not exist.

Consider this. If I put a scale at the bottom of a pool, you will weigh less inside the pool than outside according to the scale. However if I put both you and the pool onto a scale, your full weight will show.

3. Oct 11, 2012

### DrSmersh

Considering that air pushes a baloon I assume that it is a fluid, I mean it has the bouyancy effect on everything in the atmosphere. We might not float but we weight less (like a rock under water) if we compare us standing on a scale in a similar planet (equal gravity), is that correct?
So in all the exercises that i did when practicing Newton's laws the result in real life should be different partialy because of this airpush? I suppose the difference would be very small then, but does a civil engineer have to take that in account when building a bridge or any structure for example? Or a phyisist when trying to measure the as close to real as possible mass of a particle or element (I don't know how they measure this things but suposing that in some part of the process they have to weight something and decompose the forces that are presented)?

Last edited: Oct 11, 2012
4. Oct 11, 2012

### Staff: Mentor

Mass is measured differently, so we don't really have to worry about that. Plus, if you REALLY want to measure the weight of an object, just take it into a vacuum chamber. In any case, the real world differences are so tiny that in practically all cases you don't need to take them into account in order to build anything. (Unless you are trying to exploit the effect of course)

5. Oct 11, 2012

### DrSmersh

Awesome thanks, that answers my question.