The force of atmospheric pressure, like so many other concepts in physics, greatly challenges our common sense notions. One of the early investigations into this force was the Magdeburg Hemispheres experiment in the 1600's (see Wikipedia) where teams of horses could not pull apart two copper hemispheres held together only by atmospheric pressure, a high vacuum having been created between them.
In regard to your question it is helpful to consider two additional aspects besides those already mentioned above :-
- Why do our senses not register any `sensation' of the force of the atmosphere? After all, on placing a 1kg weight on 1cm##^{2}## on our skin we would definitely feel a very distinct sensation of pressure, yet the atmosphere already exerts such a force at all times, and on every 1cm##^{2}## sq of our whole body.
- If atmospheric pressure is acting all around the very complicated shape of our body, and it amounts to actual tons of weight (approx 1 tonne on every 30 x 30cm square), then could this not affect our mechanical stability? In classical mechanics if we drew a force diagram, we would be interested in the net external force and the net external torque acting on the object - is it obvious that these are both zero for the contribution of atmospheric pressure? We might surmise it might be, and in fact we can prove this for any uniform pressure field. The equalization of pressure from inside our bodies involves internal forces, and these do not enter into the two conditions of equilibrium in mechanics.
Aspect (1) :-
This brings into question how we actually perceive forces in general. It is clear we are highly sensitive to `contact' forces, which are ultimately forces between molecules. What our nerves detect is
pressure acting on the tissues of our body, ie. force ##\div## area, and so for example a moderate force over a very small area (eg a knife edge or pinprick) will register as a very `strong' signal to our brain. From the moment our life began as a single cell we have been inside atmospheric pressure, and though our nervous system could detect this pressure, it simply has no reason to send a signal to our brain prompting us to action - this is our normal environment for which we are designed. Likewise a deep sea fish does not register the high pressures it is accustomed to, though it would be sensitive to higher pressures, eg. if it came into contact with a sharp object. Throughout the millions of years of evolution all lifeforms have been accustomed to their natural environment, which in some cases such as tardigrades is
very extreme - their nervous systems are designed to register only variations from their normal environment. It is an interesting fact to note in the passing that these contact forces of pressure appear to be the only forces we can
directly sense, in particular we cannot directly sense the force of gravity (for example astronauts in the ISS experience exactly the same sensation of `weightlessness' as they would do if traveling in a spacecraft at constant velocity in interstellar space away from any sources of gravity). (In fact this was one of the key insights of Einstein that led him to his general theory of relativity - eg see Wikipedia articles on Weightlessness, and Ficticious Forces). In keeping with our perception of atmospheric pressure as a kind of base or zero level of pressure, the notion of `gauge pressure' means pressure
above ambient atmospheric pressure, eg tyre pressure (when its completely flat the air inside it is
at atmospheric pressure, so there is no net force on the tyre walls), or blood pressure (eg a systolic blood pressure of 120 mmHg means 120 mmHg
above atmospheric pressure (the latter is of the order of around 750 mmHg) - the heart's pumping action is largely responsible for the additional 120 mmHg of pressure).
Aspect (2) :-
In solving mechanics problems, unless we are dealing with fluids, we never usually stop to think about the atmospheric pressure forces acting on the body, and they don't appear in our force diagram, even though these are very large forces. This is because in a
uniform pressure field, such as for everyday objects on the surface of the earth, net
external force of pressure is zero, and wrt any reference point the net
external torque of the pressure forces is zero, so we can completely disregard the effect of pressure. To prove the former, we can use the fact that the net force of a uniform pressure on a surface in a particular direction is the pressure multiplied by the area of the
projection of the surface onto a plane perpendicular to the direction, eg as calculated for a spherical soap bubble when considering the notion of surface tension. The pressure forces on the closed surface ##S## of a body in a particular direction must then sum to zero. (For more complicated shapes we can piece them together from simpler shapes, in a similar way as we do with Gauss's and Stokes' Theorems). Thus the net vector force is zero, as the above applies to any direction. To prove the net torque of the pressure forces is zero, choosing an arbitrary reference point ##O##, we can write the total torque as :- $$\tau = -p \iint_{S} \mathbf{r} \times \mathbf{n} \, dS$$ where ##p## is the uniform pressure, ##\mathbf{n}## is unit outward normal to ##S##, and ##\mathbf{r}## is the position vector of a surface element wrt ##O##. We can write this out one component at a time, and express it as a volume integral using Gauss's Theorem (ie Divergence Theorem), which is then readily shown to be zero.
A couple of interesting home experiments that illustrate atmospheric pressure are (i) drill a hole (say 5-8mm) near bottom of a plastic bottle, fill it up and screw cap on - water will
not flow out the hole! Unscrew cap slightly and it will flow immediately. Screw cap back on again and it will stop flowing! (ii) upturned glass of water and card experiment - search online, a remarkable phenomena, though requires a steady hand.
Both (i) and (ii) work by the same principle - the air cavity at the top
expands slightly (only by about 1% or so, so it is not visible), causing its pressure to drop (by Boyle's Law) which then causes equalization of the pressure at the bottom of the water column to the external atmospheric pressure.
Ravi Singh choudhary said:
So it is due to larger surface the same atmospheric pressure is able to collapse such huge container.
Yes the area is crucial, for example a small and a large boat may both sink into water the same depth, so that the pressure experienced is the same, but the bouyancy force on the large boat is much greater due to the larger surface area, so its greater weight can be supported. With a `gas spring' it is area difference at either side of the piston head that produces the force (the pressure is the same at either side).
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