Question about a Pair of Forces - Newton's Third Law

[ProPM]

I was looking at a picture (attachment) in a physics website and the following doubt arose:

I previously thought that the weight of the block (gravitational pull of the Earth on the block) caused a reaction force of the table, but now, from what I understand and, please correct me if I am wrong, those are not Newton's pair of forces because the weight is a force exerted by the Earth ON the block and the reaction force of the table is also ON the block, thus, two forces acting on the same body.

But, the diagram shows a second pair of forces it calls the normal reaction force due to the contact: I am not sure, but I remember my teacher saying that the nature of the normal reaction force is that as the block stands on the table it pushes some molecules of the table down and the molecules in turn push up. I am confused with that, because, if that really happened, wouldn't it be due to the weight and then the same problem of two forces acting on the body occur? So is that the nature of what the picture calls the normal reaction force due to contact?

Any help appreciated,
Thanks in advance,
ProPM.

 

[cupid.callin]

Hi ProPM

The picture is correct

The Newton's action rxn pair act on 2 different bodies and thus the weight and normal rxn is not Newton's action rxn pair

Here in pic, Earth and block attract by force mg
the block when placed on table (as we know) do not move down, so ner force on it must be 0

the upward force of magnitude mg comes from table ...
Your teacher is correct about this ... the normal force is electromagnetic, the 2 layers repel each other and the block is in equilibrium from table at a distance such that this normal force becomes equal to mg, (thus block stays at rest)

And according to The Newton's action rxn law, if table pushes block with a force F, block will also push table with mg in opposite direction (ie mg in this case downwards)

This pair is called action rxn pair

 

[ProPM]

Cool, thanks!

So, just to make sure, the normal contact force is due to the contact of the box with the table because the Earth is pulling the box down on the table and the magnitude of the two forces shown in the new attachment are also mg?

Thanks a lot, sorry, I'm a bit insecure

Thanks a lot Cupid.Callin,
ProPM

 

[cupid.callin]

So, just to make sure, the normal contact force is due to the contact of the box with the table because the Earth is pulling the box down on the table and the magnitude of the two forces shown in the new attachment are also mg?

Yes both the N are mg.

(Note: do not think it will be same for the table also, i mean when block is placed on table of mass M, the floor wont exert Mg force on table... but i guess you can leave this concept for now, get back on this thread if you need help )

 

[rwisz]

I can provide some clarification and explanation on this topic. First, it is important to understand that Newton's third law states that for every action, there is an equal and opposite reaction. This means that whenever a force is exerted on an object, there is always a force of equal magnitude in the opposite direction.

In the case of the block on the table, the weight of the block is a force exerted by the Earth on the block, and according to Newton's third law, there must be an equal and opposite reaction force. This reaction force is the normal force exerted by the table on the block. This normal force is what prevents the block from falling through the table and is also responsible for supporting the weight of the block.

You are correct in understanding that the weight of the block and the normal force are not a Newton's pair of forces, as they are not acting on different bodies. However, they are still an example of Newton's third law in action.

Now, regarding the nature of the normal force, your understanding is partially correct. The normal force is indeed a result of the molecules in the table pushing back against the molecules in the block, but it is not solely due to the weight of the block. This force is also influenced by the properties of the surface of the table and the block, such as their roughness or smoothness.

In summary, the weight of the block and the normal force are not a Newton's pair of forces, but they are still an example of Newton's third law. The normal force is a result of the interaction between the molecules of the block and the table, and it is not solely due to the weight of the block. I hope this helps clarify your doubts.