Net force on 'ground' with object on top of it....?

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Discussion Overview

The discussion revolves around the forces acting on a stationary object resting on the ground and the implications for the ground itself, particularly as one considers the scenario extending to the center of the Earth. Participants explore concepts related to weight, normal forces, and the behavior of materials under pressure.

Discussion Character

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

Main Points Raised

  • Some participants note that a stationary object experiences both a weight force and a normal reaction force from the ground, which sum to zero.
  • Questions are raised about what force acts on the ground to prevent it from experiencing a net downward force, with speculation about the normal force from the ground below.
  • One participant suggests that as one moves toward the center of the Earth, the normal reaction force of an infinitesimally thin piece of ground may change, potentially leading to infinite values at the center.
  • Another participant discusses the implications of gravitational attraction and the volume of objects, suggesting that compression forces may prevent further clumping of mass.
  • There is a mention of how pressure affects the state of materials, with a claim that under pressure, everything becomes fluid, contributing to the spherical shape of planets.

Areas of Agreement / Disagreement

Participants generally agree on the initial forces acting on the object and ground, but there is disagreement and uncertainty regarding the implications of these forces as one considers deeper layers of the Earth and the nature of normal forces at those depths.

Contextual Notes

Participants express uncertainty about the behavior of forces at the center of the Earth and the relationship between gravitational forces and material properties under pressure. The discussion does not resolve these complexities.

orthogonal1
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A stationary object resting upon the ground experiences two forces, the weight force as well as the normal reaction force of the ground. These forces sum to zero. The ground however experiences the weight force acting downwards upon it. What other force acts upon the ground to prevent it from experience a net force downwards? Is it the normal force of the ground immediate below? If we continue this until we get to the centre of the earth, what is the normal reaction force of an infinitesimally thin piece of ground immediately about the centre of the Earth?
 
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orthogonal1 said:
A stationary object resting upon the ground experiences two forces, the weight force as well as the normal reaction force of the ground. These forces sum to zero. The ground however experiences the weight force acting downwards upon it. What other force acts upon the ground to prevent it from experience a net force downwards? Is it the normal force of the ground immediate below? If we continue this until we get to the centre of the earth, what is the normal reaction force of an infinitesimally thin piece of ground immediately about the centre of the Earth?
What is the normal force on the hub of a bicycle wheel from all of the spokes? What is it about the bicycle wheel that allows such a force to exist? Is there a similar effect that you are not accounting for within the Earth?
 
Good reasoning. And it leads to something strange, so there must be a flaw somewhere. Because if you come from the other side of the Earth the forces are pointing the other way!
One of the flaws makes things worse: the further down you go, the smaller the area of the remaining sphere that has to deliver all this normal force ! So would that force become bigger and bigger, to even go to infinity at the very center ?

Gravity makes things with mass attract each other. But things have volume, so at some point the opposite force (opposition to compression) prevents further clumping together and disappearing into a zero volume.

As they say: under pressure everything becomes fluid. That's the reason planets are spherical. If they were cubic, the corner tips would ecxercise so much pressure on the material underneath that the latter would give way until things even out.
 
orthogonal1 said:
What other force acts upon the ground to prevent it from experience a net force downwards? Is it the normal force of the ground immediate below?
Yep.
If we continue this until we get to the centre of the earth, what is the normal reaction force of an infinitesimally thin piece of ground immediately about the centre of the Earth?
Keep going! The line of logic you are on is working fine.
 
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