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If I push a box forward, then probably, the force I apply can be considered external force acting on the system of particles of the box. In rigid bodies like the box, each particle of the body should experience some force and as a result of the interaction of applied force with each particle of the body there is a net external force which will move the object forward. (Do correct me if I'm wrong.)

But I kept wondering that if I'm applying the force to certain portion of the box then the forces will be experienced only by the particles directly in contact with my hands. Then how does other particles of the rigid object experience force? Is it related to intermolecular forces?

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  • $\begingroup$ In your first para you said all particles could 'experience' the force. Why does it seem contradictory then? $\endgroup$
    – QuIcKmAtHs
    Commented Jan 1, 2018 at 3:42
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    $\begingroup$ That's not what I said but rather that's what I read. And on thinking about a general case of pushing an object and trying to use the concept I came across at the very beginning of chapter "Momentum" i.e about system of particles to explain the interaction of of source with each particle of rigid body, I got confused on how the particles inside the object really experiences the force I'm exerting in just one portion of the object on its outside. $\endgroup$
    – suiz
    Commented Jan 1, 2018 at 4:36
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    $\begingroup$ You have it right. The electrons at the surface of your hand interact and perturb the electrons at the surface of the box. The perturbed electrons apply a force to their nuclei, and the first layer of atoms is perturbed. The perturbation of the first layer changes the net force on the second later, and so on. $\endgroup$
    – garyp
    Commented Jan 1, 2018 at 5:01

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At the instant you apply a force onto a part of the body, the intermolecular electrostatic forces will cause the force to propagate along the body. One could think of it as a 'ripple effect' propagating through the solid Now if the force is applied such that the line of action of the force does not pass through its center of mass, then the body will also experience a torque, or a turning effect. If the torque is sufficient, you may even observe that the block topples while you push it.

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First of all, rigid body is an assumption. There is no real rigid body exists.

If you consider box as a rigid body, you are right, the external force will be distributed among all the particles of the system in such a way the acceleration of all the particles of the system is same.

But, I think from your question, it looks like you want to know about the real object and not just an ideal rigid body. In that case, the body will experience some deformation, elongation, shear etc. If you mean to know answers to that I would say that there is course of "Mechanics of Solids (MOS)" in the first and second year of the Mechanical engineering. If you want to study this, I could suggest you some good books.

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