Pressure

Date: 2015-10-07; view: 431.

Weight

Air has its own weight. It's quite easy to demonstrate this by weighing an empty balloon on a set of accurate scales. Then pump the balloon up with a bicycle pump and weigh it again. The balloons got heavier. The reason it's got heavier is because the balloon is now full of compressed air. If air didn't have any weight, then the balloon with compressed air in it would weigh the same as the empty balloon. Because it's heavier, we've demonstrated that air has weight.

Because the weight of air varies with pressure and temperature it has to be defined accurately.

Air is under pressure; this is caused by gravity. If gravity didn't exist, then air pressure would not exist. Air would also be weightless, although it would still have mass.

Air pressure at sea level is approximately 1013 mbar, which is about the same as 14.7psia (pounds per square inch, absolute).

The reason for this pressure is because there is so much air stacked up on top of it. If you were higher up, say in an aeroplane, the air pressure outside the 'plane would be much lower.

We know that the air pressure at 18,000 ft (about 5500 metres) is approximately half that at sea level.

At 32,000 ft (about 10,000 metres) the air pressure is only a quarter of that at sea level.

The reason for the reduction in pressure is because there is less air stacked up on top at these high altitudes.

Just a quick note to explain the difference between mass and weight. Here on earth we assume weight and mass to be the same, in our normal day to day lives we don't need to appreciate the difference.

If you were up in space and floating around weightless, you would quickly appreciate the difference. Imagine launching two satellites from earth, each weighing a couple of tonnes. Once up in space these satellites would be weightless. If something went wrong and they bumped into each other and you were between them, you'd get squashed. Even though they are weightless in space, they retain their mass - and it's the mass that squashed you.