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Consider the case where there is a block A on another block B.

There are many forces acting between them. There is $mg$ by A down and then $mg$ by Earth upwards.

Why is $mg$ of A not equal to $Mg$ of A plus $Mg$ of B?

Additionaly, we have many normal reaction pairs. The force of Earth on block B and the force of block B on Earth.We also have the force of block B on A and the force of A on block B.

I have checked many videos that we have to consider forces that act on the body. The thing is, even the weight of A is acting on B. Why don’t we consider it?

I am getting confused because of Newton’s third law and how to make correct system and FBD for a body. No one on Internet has given correct information about the things that you do not include in FBD’S. It is getting really confusing. I want to understand which forces are to be included when considering a system and making a FBD.

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I want to understand which forces are to be included when considering a system and making a FBD.

Free body diagrams only show the forces acting on that body. The do not show forces acting on other bodies.

Why is $mg$ of A not equal to $Mg$ of A plus $Mg$ of B...The thing is, even the weight of A is acting on B. Why don’t we consider it?

Because the weight of A is only $m_Ag$. The presence/influence of B does nothing to change the weight of A. The weight of A only depends on its own mass and the acceleration due to gravity.

Additionally, the weight of an object only acts on that object, it does not act on other objects. Now, there can be other forces in the system that arise because of the weight of something (like here, the force of interaction between the blocks), but that is not the same thing as the weight of one object acting on another object.

Additionaly, we have many normal reaction pairs. The force of Earth on block B and the force of block B on Earth.We also have the force of block B on A and the force of A on block B.

Yes, in a system there can be many interaction forces. But don't be overwhelmed by them. Just look at a single object and only consider forces acting on that object.

For A you have

  1. The weight of A downward
  2. The force of B on A upward

For B you have

  1. The weight of B downward
  2. The force of A on B downward
  3. The force of the surface (whatever B is resting on) on B upward
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  • $\begingroup$ When you try pick A , you will feel the force of both A and B right ? $\endgroup$
    – Srijan
    Commented Dec 10, 2020 at 15:29
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    $\begingroup$ @robertpatrick I don't understand your question. What is "the force of A" or "the force of B"? $\endgroup$
    – BioPhysicist
    Commented Dec 10, 2020 at 15:30
  • $\begingroup$ I meant the mass of A will be total mass of A + B $\endgroup$
    – Srijan
    Commented Dec 10, 2020 at 19:30
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    $\begingroup$ @robertpatrick No. The mass of A is just the mass of A. If you are drawing the free body diagram of block A, then you only consider the forces acting on block A. $\endgroup$
    – BioPhysicist
    Commented Dec 10, 2020 at 19:30
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    $\begingroup$ @robertpatrick I have already said what you include for each block in my answer. $\endgroup$
    – BioPhysicist
    Commented Dec 10, 2020 at 19:34
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What you should understand is that when an object applies force on another object , there is an action reaction pair.After that , Thr force on both objects is same but not the acceleration and mass maybe.So depending on that , we only write the forces that were forced on other body to cause its acceleration.

Also in FBD , we don’t include forces of other objects on the body.Only forces which acted on the body and cause its own acceleration

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