Action-reaction pairs, concerning gravitational force &normal reaction

In summary, the conversation discusses the concept of being at rest on the surface of the Earth and the role of action-reaction pairs and forces in this scenario. It is clarified that the normal reaction force from the ground does not cancel out the gravitational force, but rather produces a net force of zero. This is due to the fact that action-reaction forces act on different bodies. The conversation also delves into the specific forces acting on an individual and the ground in this situation.
  • #1
FieldvForce
30
0
I would like to know how it is possible that I can be at rest on the surface of the Earth (relative the the Earth of course).

My main cause for confusion is the action reaction pair that occurs when I stand on the surface of any solid material.

The normal reaction nornal reaction a-r pair ensures that I don't pass through the ground and that the ground doesn't pass through me, however would that not mean that there is a resultant force greater 0.

I'll elaborate, say I weigh 80kg and so I am being pulled toward the centre of the Earth with a force of just under 800N, the Earth is being pulled toward me with a force of 800N also, though my acceleration is greater than that of the Earth's; the normal reaction - normal reaction a-c pair cancels out and the Earth and I approach the other's center with a resultant force of 1600N between us, yet I remain at rest, why is this?
 
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  • #2
You remain at rest--unaccelerated--because the net force on you is zero. The forces on you are the gravitational pull of the Earth pulling you down and the normal force from the floor pushing you up. These forces are not 'action/reaction' pairs.
 
  • #3
Doc Al said:
You remain at rest--unaccelerated--because the net force on you is zero. The forces on you are the gravitational pull of the Earth pulling you down and the normal force from the floor pushing you up. These forces are not 'action/reaction' pairs.

There is a action reaction pair between myself and the ground, the normal reaction from myself that results from the normal reaction of the ground, this pair itself is brought about indirectly by gravity in this particular situation.
 
  • #4
FieldvForce said:
There is a action reaction pair between myself and the ground, the normal reaction from myself that results from the normal reaction of the ground, this pair itself is brought about indirectly by gravity in this particular situation.
Yes, there is an action-reaction pair: You push down on the ground and the ground pushes up on you. What about it? (Neither of those is a gravitational force, of course.)
 
  • #5
Doc Al said:
Yes, there is an action-reaction pair: You push down on the ground and the ground pushes up on you. What about it? (Neither of those is a gravitational force, of course.)

Well I just meant surely that action reaction pair is what is responsible for the resultant force of 0N.

What remains is the resultant force of 1600N bringing my centre and the Earths centre together.
 
  • #6
FieldvForce said:
Well I just meant surely that action reaction pair is what is responsible for the resultant force of 0N.
No, not at all. Action/reaction forces act on different bodies. Only one of those forces acts on you, so you need another force acting on you--gravity--to produce a net force of zero.

Action/reaction pairs, since they act on different bodies, never 'cancel out'.
 
  • #7
Doc Al said:
No, not at all. Action/reaction forces act on different bodies. Only one of those forces acts on you, so you need another force acting on you--gravity--to produce a net force of zero.

Action/reaction pairs, since they act on different bodies, never 'cancel out'.

Ah, thanks a lot that really quashes the paradoxical notion of stationary objects obeying Newtons third law but not his first.

Just to delineate.

Gravity is pulling me towards the center of the Earth, as a result I meet with the ground (without sufficient KE to break through) with a force of 800N.

The normal reaction of the ground is 800N and so the net force acting on me is 800 - 800N

The normal reaction normal reaction a-r pair is comprised of my normal reaction against the ground (so that it doesn't pass through me) and it's normal reaction against me (so that I don't pass pass through it).So taking down as the positive direction.We have F (me) =800N (i.e mg) - 800N (i.e normal)
We have F (ground) = not 0 or?
 
Last edited:

1. What is an action-reaction pair in relation to gravitational force and normal reaction?

An action-reaction pair is a pair of forces that act on two different objects and are equal in magnitude but opposite in direction. In the case of gravitational force and normal reaction, the action-reaction pair refers to the force of gravity pulling an object towards the center of the Earth and the equal and opposite force exerted by the surface of the Earth on the object, known as the normal reaction force.

2. How does the normal reaction force counteract the force of gravity?

The normal reaction force is exerted by a surface on an object and is perpendicular to that surface. This force is a result of the intermolecular forces between the surface and the object. When an object is placed on a surface, the normal reaction force is equal in magnitude and opposite in direction to the force of gravity acting on the object. This counteracts the force of gravity and prevents the object from falling through the surface.

3. Are action-reaction pairs always present in the interaction between an object and the Earth?

Yes, action-reaction pairs are always present in the interaction between an object and the Earth. This is because every action has an equal and opposite reaction according to Newton's Third Law of Motion. The force of gravity and the normal reaction force are examples of action-reaction pairs in this interaction.

4. How does the magnitude of the normal reaction force change in different situations?

The magnitude of the normal reaction force depends on the weight of the object and the angle at which it is placed on the surface. If an object is placed on a flat surface, the normal reaction force will be equal to the weight of the object. However, if the surface is tilted, the normal reaction force will be less than the weight of the object, as it is now acting at an angle.

5. Can an object experience a normal reaction force without the presence of gravity?

No, an object cannot experience a normal reaction force without the presence of gravity. The normal reaction force is a result of the force of gravity pulling the object towards the Earth's surface. If there is no gravitational force acting on the object, there will be no normal reaction force from the surface.

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