Why are 10kg heavy?
February 8, 2013 4:04 PM Subscribe
We carry about 1,000kg of air on top of our shoulders. Why do we feel noticeably heavier when adding a fraction of it, like 10kg? Are our muscles just finely tuned?
Just because there's a huge column of air above us doesn't mean we're "carrying" it all. The amount of force from air on the top of my head is about the same as the force on the bottom of my chin. They balance out.
If you then put a stack of books on top of my head, that would not balance out, and I'd have to actually work to hold them up.
posted by aubilenon at 4:12 PM on February 8, 2013 [2 favorites]
If you then put a stack of books on top of my head, that would not balance out, and I'd have to actually work to hold them up.
posted by aubilenon at 4:12 PM on February 8, 2013 [2 favorites]
Air doesn't push straight down, as a dumbbell would. Air pushes in all directions, including up, so the force is largely balanced out.
posted by 2bucksplus at 4:14 PM on February 8, 2013 [1 favorite]
posted by 2bucksplus at 4:14 PM on February 8, 2013 [1 favorite]
Also the air is being held up by... the rest of the air. The issue is the air pressure, not just the column of matter directly above you. When you're at two atmospheres of pressure underwater (about 10 feet, right?) the pressure pushes evenly across your body not just in the direction of gravity.
posted by GuyZero at 5:05 PM on February 8, 2013 [2 favorites]
posted by GuyZero at 5:05 PM on February 8, 2013 [2 favorites]
Read about buoyancy [wiki]. Because we are more dense than air, we "sink" to the ground, and we experience pressure from the air above us, but our bodies have evolved to withstand that pressure. It's similar to swimming to the bottom of a pool; you feel more pressure but you aren't crushed by the weight of the water above you.
posted by Durin's Bane at 5:08 PM on February 8, 2013
posted by Durin's Bane at 5:08 PM on February 8, 2013
Pressure !/= weight.
posted by valkyryn at 5:25 PM on February 8, 2013 [1 favorite]
posted by valkyryn at 5:25 PM on February 8, 2013 [1 favorite]
The other posters have answered the question, but I thought I might add an example to illustrate their answers. Your main issue is that you're thinking that pressure is pushing DOWN, and that DOWN is somehow a "preferred" direction for that force to be pushing. But consider someone in the International Space Station, which is in microgravity. There is no preferred DOWN direction, and yet there is pressure. If there is X pressure "pushing down" on someone's shoulders, one could just flip over, and that same pressure is exerted in the other direction relative to their body. But nothing has changed, because the pressure is being exerted in all directions on the astronaut's body.
Or consider a balloon at sea level. A balloon is round, because the pressure is equal on sides of the balloon; if DOWN were a preferred direction, the balloon would be flattened. Since the pressure on all sides of the balloon is the same as the pressure on the top, the balloon is not flattened, but round.
posted by Philosopher Dirtbike at 3:22 AM on February 9, 2013 [3 favorites]
Or consider a balloon at sea level. A balloon is round, because the pressure is equal on sides of the balloon; if DOWN were a preferred direction, the balloon would be flattened. Since the pressure on all sides of the balloon is the same as the pressure on the top, the balloon is not flattened, but round.
posted by Philosopher Dirtbike at 3:22 AM on February 9, 2013 [3 favorites]
When you're at two atmospheres of pressure underwater (about 10 feet, right?)
ten meters. Every ten meters adds one atmosphere.
posted by DreamerFi at 8:36 AM on February 9, 2013
ten meters. Every ten meters adds one atmosphere.
posted by DreamerFi at 8:36 AM on February 9, 2013
This thread is closed to new comments.
posted by ssg at 4:10 PM on February 8, 2013 [11 favorites]