A Possible Theory for How
Using what we know about particle motion, particle collisions, and the forces on walls, we are finally ready to put forward a possible theory for how the plunger inside a syringe moves up or down when no one is touching it.
Every time an air particle inside the syringe bounces off of the bottom of the plunger, it pushes the plunger up. Each of those pushes causes the plunger to change speed and/or direction. And if there are no other forces acting on the plunger, then each of those little pushes would cause the plunger to start moving up, faster and faster.
On the other hand, if we look at the top of the plunger, there are air particles around the outside of the syringe that are also bouncing off of the plunger. Every time one of those air particles bounces off of the top of the plunger, it pushes the plunger down. And if there are no other forces acting on the plunger, then each of those little pushes would cause the plunger to start moving down, faster and faster.
So one possible theory for how the plunger moves on its own is that it is being pushed, either from the top or the bottom, by air particles. Keep in mind that this is still just a theory and it hasn’t been tested yet through predictions (hypotheses) and experiments. But it is a reasonable theory and it makes sense based on what we know.