# How does the atmosphere affect human weight on Earth?

1. Sep 23, 2011

### blarznik

Does Earth's thick layer of atmosphere push down on us, increasing our weights drastically?

2. Sep 23, 2011

### D H

Staff Emeritus
It decreases our weight, very slightly.

3. Sep 23, 2011

### blarznik

So humans are slightly buoyant? If I weighed 150 pounds would it be relieving less than a pound?

Also how thick would our atmosphere have to be to have it exert pressure on us? For instance like how at certain depths water pressure starts to overcome buoyant forces on an object.

Last edited: Sep 23, 2011
4. Sep 23, 2011

### D H

Staff Emeritus
Exactly.

Let's assume you have a density of 1 g/cc. Some people float, some sink, so 1 g/cc is about right. Air at sea level has a density of about 1.2 kg/m3, so that means you displace about 2.9 ounces of air.

5. Nov 28, 2011

### Allenbon

It it true that in different parts of the Earth the weight of a particular human is different?

6. Nov 28, 2011

### D H

Staff Emeritus
That depends on what you mean by "weight" and what kind of scale you use to measure it.

The scale you step on in a doctor's office uses gravity to balance two objects. These scales measure mass. Whether that doctor's scale is used at the north pole, the equator, or some hypothetical station on the Moon, it will register more or less the same "weight". Here, weight is being used in the legal (in the US) and colloquial (almost everywhere) sense: Weight is a synonym for mass, and has units of mass.

The scale you step on in your bathroom measures the force needed to offset the force due to gravity on a rotating Earth. Take that scale to the north pole, the equator, or that hypothetical station on the Moon, it will register different values for your "weight". Here, weight is being used in the sense of what some call scale weight. This weight is a force and properly has units of force. Your bathroom scale displays this force in units of mass assuming that gravitational acceleration is 9.80665 meters/second/second.

The Earth is rotating, so this adds a centrifugal force component (directed outward, so subtractive) to the force measured by a bathroom scale; this varies from zero at the poles to a maximum at the equator. The rotation also makes the Earth have an equatorial bulge, so someone on the equator is further from the center of the Earth than is someone at the north pole. Both effects reduce scale weight slightly at the equator compared to at the poles. For our 150 pound (mass) person, the effect is to reduce the scale weight by about 12.7 ounces-force.

7. Nov 28, 2011

### Allenbon

Thank you D H, now it is clear.

8. Nov 29, 2011

### Nik_2213

"or some hypothetical station on the Moon, it will register more or less the same "weight".

If it is a classic 'balance' with counter-weights, sure, it measures mass minus relative densities' buoyancy correction. But, if it is a spring or force-feedback device then the weight displayed will vary according to local gravity.

D'uh, we had to fully re-calibrate laboratory balances if we moved them to a taller bench, never mind down to the production area...

9. Mar 25, 2012

### tal444

It is always exerting pressure on us, we are simply adapted to living in this atmosphere.