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When you throw a ball in the air it is being pulled down by gravitational force, so we can take gravity as the force and the ball as the surface. However, it seems that this isn't pressure, but why not?

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  • $\begingroup$ Do you know want pressure is? Force and pressure are defined differently. That's the answer. $\endgroup$
    – Steeven
    Commented Jan 22, 2017 at 18:56
  • $\begingroup$ I know pressure is the force we exert on or against something? $\endgroup$ Commented Jan 22, 2017 at 19:00
  • $\begingroup$ I see the misunderstanding now, I believe, thank you for clarifying. Please see my answer. $\endgroup$
    – Steeven
    Commented Jan 22, 2017 at 19:11

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The answer is: "pressure" is force per area.

In other words: Pressure is the word used for "force per square meter" for example.

  • When sitting on a chair, a normal force $F$ holds you up. It could be 500 Newton.

  • Divide this with the area $A$ of the contacting surfaces, and you have something we could call the normal pressure $p=F/A$. This simply says something about how "spread out" the force is over the chair. If the chair was half a square meter large, $A=0.5 \;\mathrm{m^2}$, the pressure would be $p=500\;\mathrm{N}/0.5\;\mathrm{m^2}=100\;\mathrm{N/m^2}$.

A 50 kg woman in high heels might break through the grass lawn and sink in with the heels, because the heel area is tiny compared to a flat shoe, even though the lawn carries the same weight.

The point of pressure is knowing how "spread out" the force is.

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  • $\begingroup$ So pressure acts like an adjective for force? Describes how narrow/wide it is? $\endgroup$ Commented Jan 22, 2017 at 19:12
  • $\begingroup$ @OkamaKsakas Adjective? It tells how "spread out" a force is, yes. But all of course depends on situation, and "narrow/wide" is not very specific. The air inside a balloon also puts some total force on the walls, which corresponds to a pressure if you divide by surface area. $\endgroup$
    – Steeven
    Commented Jan 22, 2017 at 19:14
  • $\begingroup$ What I thought when about the grass scenario: prntscr.com/dyy9v0 so the heel one may break but the flat ones wont break. But what's the reason? and I don't get what do you mean by force per area, does that mean force exerted on an area?I imagine a shoes pressing down on the ground would be pressure, as the surface would the ground and the force is coming from the shoes $\endgroup$ Commented Jan 22, 2017 at 19:25
  • $\begingroup$ @OkamaKsakas A shoe stepping on the ground exerts force and exerts pressure. Those are two words of explaining the same push. "per area" just means "per square meter" for example. I have updated my answer a bit, to clarify. Does this make it less confusing? $\endgroup$
    – Steeven
    Commented Jan 22, 2017 at 19:52
  • $\begingroup$ Lel, I am so stupid I gotchya now! It's the force exerted on a certain surface. Thanks. $\endgroup$ Commented Jan 22, 2017 at 20:00
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Just like electrostatic pressure in a charged shell, there will also be a gravitational pressure acting on the surface of the ball (but very negligible). The force considered between the earth and the ball (of negligible dimensions in comparison to earth) has an overall zero effect on the surface because of the negligible variations in the field w.r.t the ball. So, force gives the molecules of balls a uniform direction-ed motion , with no randomization among them .Hence no pressure due to earth's gravitation on ball. If the ball would had been of comparable size , pressure would had been appreciable.

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  • $\begingroup$ I didn't understand a single word of what you said. Can you please explain it more simply? All I know is that when there's a force acting on or against the object it's called pressure. However, someone told me we don't call this pressure, I am confused. $\endgroup$ Commented Jan 22, 2017 at 14:18
  • $\begingroup$ Pressure is not exactly force on an object (it can vary from thermal pressure to quantum degeneracy pressure). Classical pressure is actually the average normal force per unit area on the surface. Here the average force normal to surface on the ball cancels (due to assumption of uniformity in the field). $\endgroup$
    – Anand
    Commented Jan 22, 2017 at 14:21
  • $\begingroup$ What do you mean by "the average force normal to surface on the ball cancels (due to assumption of uniformity in the field)"? $\endgroup$ Commented Jan 22, 2017 at 14:28
  • $\begingroup$ take an infinitesimal element on the surface of the ball and then take component of gravitational force along the outward normal of that surface element. The upper hemisphere will have effective downward force (opposite to direction of outward normal (-ve (compressive) pressure), whereas the lower hemisphere has effective +ve (elongative) pressure. Hence cancellation. $\endgroup$
    – Anand
    Commented Jan 22, 2017 at 14:35
  • $\begingroup$ I got you until the last part what do you mean by "whereas the lower hemisphere has effective +ve (elongative) pressure." $\endgroup$ Commented Jan 22, 2017 at 15:00
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Your main problem is that you're not keeping concepts straight in your head, and apply names willy-nilly to things that have no connection to the concept referred to by a word.

Specifically, "gravity" refers to a phenomenon, not a force, and a "ball" is not a surface. To mention more examples from your comments on your other post ("So when I pull a chair this is force. When I sit on chair this is pressure?"), when you pull a chair it's called "pulling a chair", not "pressure", and when you sit on your chair it's called "sitting on your chair", not "pressure".

When you pull a chair you may do so by exerting a force on said chair by various means. For example, if the chair is made of iron, you might pull it with a magnet, in which case the force exerted on the chair is due to a magnetic field. Or, if you pull the chair in some mechanical way, it is likely that the force exerted on the chair acts on some part of its surface. In this case there will be an average pressure that is given by the ratio of the force divided by the surface area of this portion of its surface.

Now, on your ball example, there is a gravitational force that acts on each and every particle within the volume of the ball. The corresponding force is therefore also called a "volume force". The concept of "pressure", on the other hand, refers to forces acting on surfaces, that are therefore referred to as "surface forces". In the case of your ball in the air, there are surface forces due to the pressure of the fluid surrounding the ball. These forces are distinct from the gravitational force we described above, which has nothing whatsoever to do with pressure forces.

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  • $\begingroup$ How do I keep concepts straight in my head though? I want to fix that :/ $\endgroup$ Commented Jan 22, 2017 at 18:58
  • $\begingroup$ Can you tell me how to fix my main problem though? xD $\endgroup$ Commented Jan 22, 2017 at 19:09
  • $\begingroup$ Hard to tell. I would say you should try to make sure you pay close attention to the way concepts are defined in physics, and then try to stick to these definitions. In many case Wikipedia is a pretty good source. Specifically in the case of physics quite often the first sentence in a Wikipedia article gives you a good idea. $\endgroup$
    – Pirx
    Commented Jan 22, 2017 at 19:17
  • $\begingroup$ The problem is that I find them complicated to understand $\endgroup$ Commented Jan 22, 2017 at 19:27
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A ball is not a surface.   It is a group of particles (atoms or molecules).   When the Earth pulls on the ball, that force is the sum of the tiny direct forces of gravitational attraction between each individual particle in the entire Earth and each individual particle in the entire ball.   There is no surface that we can identify as where the force is applied.

Contrast this with pushing on a wall with the palm of your hand.   That force is actually the sum of electrostatic repulsive forces between the electrons in the particles at the surface of the wall and the ones in the particles at the surface of your hand.   In this case we can identify the surface where the force is applied.   The forces are transferred to deeper particles in both objects, but are directly applied only at the surface.   That surface has an area, and we can define the pressure as the amount of force per unit of area.   You can apply the same amount of force with your palm or with the tip of your finger, but the pressure will be greater when you use the tip of your finger.

Contact forces, like pushing on the wall, tension force applied by a rope, or buoyant force in a fluid, have a pressure associated associated with them given by force divided by area.   Non-contact forces like gravity, magnetism, and electrostatic force (when applied across empty space rather than at a surface) do not have pressure associated with them.

Additional Notes-- The above answer is complete, but some people may want to make connections to the ideas of fluid pressure:

Gas pressure or liquid pressure at a surface such as the inner wall of a container is the time-averaged sum of forces per unit area between particles in the fluid colliding with particles at the surface, plus the sum of repulsive forces between particles in the surface and nearby particles of the fluid.   One might ask why we can speak of the pressure in the middle of a fluid where there is no surface.   Well, you'll find that force divided by area gives the same units as energy divided by volume.   So pressure in a fluid can also be thought of as volume concentration of energy (the same kinetic and potential energies described earlier as applied between particles of the fluid and a surface).

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    $\begingroup$ A down vote, really? Would that person kindly leave a comment as to why? $\endgroup$
    – D. Ennis
    Commented Jan 22, 2017 at 15:26
  • $\begingroup$ I don't think it's you, someone's downvoting everyone for some reason. $\endgroup$ Commented Jan 22, 2017 at 19:00

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