Normal Force and Atmospheric Pressure

In summary: I remember they were for a less-advanced-type-of-building. They are made of a metal or plastic that has small holes drilled in it, and are used to create a frame for a more permanent structure. The idea is that the frame will eventually stretch and pull the plastic or metal apart, causing the structure to come down.
  • #1
Sukuya
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1. Suppose a cube of mass m is lying at rest on a horizontal surface. The area of one face of the cube is A. For the equlibrium of cube no net force acts on it. If we consider the vertical direction then the contact force N should be equal to the force due to gravity plus the force due to atmospheric pressure acting on the top face. There is no air between bottom face and the horizontal surface so atmospheric pressure is not acting on the bottom face. The atmospheric pressure is substantial too, so we can't ignore it. Still we say that N = mg instead of N = mg + PA, where P is the atmospheric pressure. Why ?
 
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  • #2
If there were actually no air between the bottom of the object and the surface, then the air pressure would be significant. In fact, that is how suction cups work (there isn't "no air" under a suction cup, but the pressure is lowered when you pull upwards on the suction cup and increase the volume of the region between the cup and the table).

This should convince you that most real objects do not seal tightly enough against the table to exclude air.
 
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  • #3
Perhaps worth a look at "Wringability" of gauge blocks. (edit...but it's not just air pressure that keeps them together).
 
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  • #4
Sukuya said:
If we consider the vertical direction then the contact force N should be equal to the force due to gravity plus the force due to atmospheric pressure acting on the top face. There is no air between bottom face and the horizontal surface so atmospheric pressure is not acting on the bottom face. The atmospheric pressure is substantial too, so we can't ignore it. Still we say that N = mg instead of N = mg + PA, where P is the atmospheric pressure. Why ?

You have the sign wrong. Atmospheric pressure buoys objects upwards rather than pushing them down. There is air underneath your block unless you take extreme measures to seal the bottom of your cube.

So why don't we use this buoyancy-corrected weight? Simple: It's a tiny effect. You ignored a couple of other tiny effects when you said "N=mg". Gravitation acceleration is not constant everywhere. You should be using N=mg(x), where g(x) is the gravitational acceleration at the location of interest. Even that is not correct because the Earth is rotating at one revolution per sidereal day. The net force on your cube has to be just the amount needed to make the cube rotate at this rate at about the Earth's rotation axis.
 
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  • #5
D H said:
You have the sign wrong. Atmospheric pressure buoys objects upwards rather than pushing them down. There is air underneath your block unless you take extreme measures to seal the bottom of your cube.

So why don't we use this buoyancy-corrected weight? Simple: It's a tiny effect. You ignored a couple of other tiny effects when you said "N=mg". Gravitation acceleration is not constant everywhere. You should be using N=mg(x), where g(x) is the gravitational acceleration at the location of interest. Even that is not correct because the Earth is rotating at one revolution per sidereal day. The net force on your cube has to be just the amount needed to make the cube rotate at this rate at about the Earth's rotation axis.

What I wasn't able to convince myself was that there was air between the block and the surface, which tantamounts to air only pushing it down. Of course the g we take is the effective g taking into consideration of Earth's rotational effects as well as variation with height/depths etc.
 
  • #6
CWatters said:
Perhaps worth a look at "Wringability" of gauge blocks. (edit...but it's not just air pressure that keeps them together).

I looked up wringability just to see what they say.
and I got more sites on Writing Ability than wringability.
I encountered the blocks only once many eons ago.
 

What is normal force?

Normal force is the force that a surface exerts on an object in contact with it. It is always perpendicular to the surface and prevents objects from passing through each other.

How is normal force related to weight?

Normal force is equal in magnitude but opposite in direction to the weight of an object. This means that when an object is resting on a surface, the normal force is equal to the weight and balances it out.

What is atmospheric pressure?

Atmospheric pressure is the force per unit area exerted by the weight of the Earth's atmosphere. It decreases as altitude increases, and is typically measured in units of pounds per square inch (psi) or pascals (Pa).

How does atmospheric pressure affect objects?

Atmospheric pressure exerts a force on objects due to the weight of the air above them. This can cause objects to compress or deform, and can also create buoyant forces on objects submerged in fluids.

How does the normal force change with atmospheric pressure?

The normal force can change with atmospheric pressure if the object is in contact with a fluid, as the fluid exerts a pressure on the object. However, in most cases, the normal force is not directly affected by atmospheric pressure and is solely determined by the surface the object is resting on.

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