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Lets consider a 2d box with nothing inside of it - just 4 walls, which consist of balls, connected by the force very similar to spring force. Look at the picture I draw. enter image description here

When the force applied to the center of the bottom surface, balls shift up and attract neighboring balls. According to this model, even if spring is very rigid, it's unclear, why the whole body is moving. Why do the top balls start to move? Again, body is empty inside; the balls from bottom can't reach the balls from top.

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3 Answers 3

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The force that pulls on the very last ball on both sides still has a component upwards of $F\cos\theta$ (shown below). In other words the entire pulling force from the adjacent ball won't be horizontal, there will be some upwards component of this pull.

enter image description here

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  • $\begingroup$ Well, don't know why I didn't think about this, seems to simply) $\endgroup$
    – Artur
    Commented Mar 11, 2020 at 15:04
  • $\begingroup$ Just a question, how did you make the diagram? $\endgroup$
    – Bandoo
    Commented Mar 11, 2020 at 15:14
  • $\begingroup$ photopea.com - Basically all the tools of Photoshop but free and online. @Bandoo $\endgroup$
    – Charlie
    Commented Mar 11, 2020 at 15:17
  • $\begingroup$ @Artur this is why you can move any (light) object with the tip of your finger, you don't have to cover the entire object's surface area with your hand to move it. $\endgroup$
    – Charlie
    Commented Mar 11, 2020 at 15:17
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If you have a string, and you push a part of it, it will oscillate back and forth. In your example, instead, the particles affected will move back and forth because they have a changing net force(initially a push forward, then as it moves up the other particles pull it down). As the particles move up and down, they rub off the particles in the air and propel the whole object forward.

If there was no air, then the particles would oscillate up and down forever. It means the affected particles have a changing net force on them, but the other ones do not, and as such they shouldn't move. Mechanical forces cannot act at a distance.

OP you didn't state in the question if the bottomleft particle could collide with the particle above it. If thats the case then yes the object will move forward.

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  • $\begingroup$ Sure, any paricles can collide $\endgroup$
    – Artur
    Commented Mar 11, 2020 at 15:00
  • $\begingroup$ @Artur Then look at the answer below, it explained what I wanted to say using a diagram. If any of the particles can collide then the whole body would truly move forward. But with that artificial limitation (that I assumed you held because of the question) the whole object in place just like a string. If you pushed that string in a vacuum it will just oscillate. $\endgroup$
    – Bandoo
    Commented Mar 11, 2020 at 15:13
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Consider all the balls together as comprising a single system. The only external force acting on the system is the arrow acting upward. That means there is a net external force on the system. Per Newton’s second law the system will accelerate.

Hope this helps

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