Question

When we stand holding a heavy object we actually feel its weight but when we run with it, we feel like it weighs less .....
i mean to say we don't feel the heaviness that way any more. Why does this happen?
.plz ansr if u knw

Answer 1

While all the answers I've seen are correct, I feel that the OP's question is something else:

Why don't we feel the weight of such a huge amount of air on us?


The main misconception here seems to be about the behaviour of air molecules and the resulting air pressure. We start off with an example:
Suppose that your palm is open, outstretched, surrounded by air. Atmospheric pressure is about 105105 Pascal. A human palm (roughly) has a surface area of about 100 sq.cm. Now, the resulting force due to the air above you, on your palm is 105∗100∗10−4N=1000N105∗100∗10−4N=1000N. That's about the same as a 100 kg block on top of your palm. So why don't we feel it? And even if we do, then why does another 1 kg block on it feel soheavy?


The answer is that the air above your palm pushes it down, but the air below your palm also pushes it up. This balances the weight, so you don't feel as if you're holding a 100 kg block. So, in a way, you are lifting up air, but air is also lifting you up. 




This might seem a bit absurd if you're learning something about air pressure for the first time. We have this false notion that since there is a lot of air above, it should be pushing us down. This is incorrect, however. Air has pressure because its molecules move at high speeds (as high as 500 m/s!) and collide with all surfaces in contact. Since they move randomly in all directions, they exert equal pressure in all directions. See Kinetic theory. If you still can't convince yourself about this, think of what is holding the air above when your palm is not there. There is only the air below to support its weight. That same air below is now supporting your palm. The same concept is applicable to your entire body, so you don't feel being pushed down by tons of air. Similarly, swimming below a ton of water is not the same as holding a ton of water on your back.

Now, we come to the other part of this question (which is not your actual query, I suppose). If air is pushing us from all sides, shouldn't our body collapse? No. This is because the blood flowing in our body provides counter-pressure. The skeleton also prevents it from collapsing.



Our bodies are accustomed to the usual atmospheric pressure of 1 atm. If the surrounding pressure is too high (like in deep sea) or too low (like in space) than this, then the forces acting on your skin from inside and outside will not be balanced, and your skin will be ruptured, often leading to death. In fact, strong winds can even rupture the delicate skin of our nose and ears and cause bleeding sometimes (mostly in babies).