What Causes the Upward Force Counteracting Gravity?

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Discussion Overview

The discussion revolves around the nature of the upward force that counteracts gravity, specifically exploring the concept of normal reaction force and the underlying mechanisms that produce this force. Participants delve into the interactions at the atomic level and the behavior of fluids under pressure, touching on both theoretical and conceptual aspects.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant identifies the upward force as the normal reaction force, which counters the downward force of gravity.
  • Another participant explains that the weight of an object compresses the atoms in the floor, leading to a repulsive force due to electrical charges in the atoms, suggesting this is often overlooked in physics education.
  • A question is raised about the uniformity of pressure in different directions within a fluid, specifically why horizontal pressure must equal vertical pressure when the net force is zero.
  • A participant provides an analogy involving oiled ball bearings to illustrate that the force exerted by a fluid on the walls of a container is consistent with the force on the base, emphasizing that pressure acts in all directions due to the weight of the fluid above.
  • Another participant seeks a mathematical demonstration of how fluids exert equal pressure in all directions, drawing a comparison to the behavior of ideal gases.

Areas of Agreement / Disagreement

Participants express varying levels of understanding and agreement regarding the mechanisms of pressure in fluids and the nature of the normal reaction force. Some points remain contested, particularly concerning the uniformity of pressure in different directions and the mathematical representation of these concepts.

Contextual Notes

Participants acknowledge that certain assumptions and definitions may be necessary to fully understand the concepts discussed, particularly regarding fluid dynamics and atomic interactions. The discussion does not resolve these complexities.

Who May Find This Useful

This discussion may be of interest to students and educators in physics, particularly those exploring concepts related to forces, pressure, and fluid dynamics.

rafterman
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I know that gravity causes the downward force, but to keep us in ballance there is an upward force.Can someone tell me what causes this upward force.
 
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normal reaction force
 
Your weight is trying to push the atoms in the floor closer together, the electrical charges in the atoms repel each other and push back.

Strangley most phyiscs classes don't seem to mention this - the reaction force is treated as just a book keeping exercise to make all the forces balance.
 
mgb_phys said:
Your weight is trying to push the atoms in the floor closer together, the electrical charges in the atoms repel each other and push back.

Strangley most phyiscs classes don't seem to mention this - the reaction force is treated as just a book keeping exercise to make all the forces balance.

I get this, but I don't understand why the pressure must be the same in every direction. If you take a small cube in a fluid at rest, the net force must be zero. So pressure must be equal on opposite sites. But why does this mean that the horizontal pressure must equal to vertical pressure?
 
That's a slightly different question - but yes.
It's difficult to immediately see why the force on the walls of the bottom of a tank of water is the same as the force on the base.
Imagine something heavy that can easily flow, like oiled ball bearings.
Now picture a large box full of them and you cut a hole in the side - you would need a lot of force to hold back the balls. The force of them coming out of a hole in the side, near the bottom, is pretty much the same as the force of them coming out of the bottom - there is still the same weight of all the balls on top of them.

The same thing applies to water or air, or any other fluid - the pressure acts in all directions because the particles are being pushed out in all directions by the weight of the fluid above them
 
I see, thank you!
 
Speaking of pressure, is there a way to show a fluid exerts equal pressure in all directions mathematically? Like for ideal gases we can use the assumption that collisions with the walls of the container are perfectly elastic to show that pV =1/3Nm<v^2> where <v^2> is the mean square speed of the particles.

Is there some other mathematics for a fluid as well?
 

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