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As the second Newton's law says $F=ma$. Why the force is increases, when I press something, if $a=0$?

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    $\begingroup$ Fhe $F$ in $F=ma$. Is the net force. If there is no acceleration it is because some other force, opposing the motion, is increasing at the same time. $\endgroup$ – M. Enns Feb 2 '18 at 19:45
  • $\begingroup$ @M.Enns, "net force"? What does it mean? I've finded a link $\endgroup$ – Артур Клочко Feb 2 '18 at 19:48
  • $\begingroup$ @M.Enns, so the other force is the electromagnetism? $\endgroup$ – Артур Клочко Feb 2 '18 at 19:51
  • $\begingroup$ The net force is the vector sum of all the forces acting on an object. Say you are pressing a book down onto a table, the net force would be the sum of the force you apply plus the force of gravity on the book plus the normal force of the table acting upwards on the book. $\endgroup$ – M. Enns Feb 2 '18 at 19:51
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The Newton's second law states that the net force is equal to the product of mass and acceleration.

I believe you are referring to the force of static friction which is Ffriction = $\mu$*FNormal

Forces on a body

Until the applied force exceeds the value of the static friction the body remains at rest. Friction force vs Applied force

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  • $\begingroup$ I almost understand, but one more question. There is a heavy ball. If I touch it, it wouldn't move, but if I touch it stronger, it will moves. As I understood, static friction force acts on it so it doesn't move by itself. So why it going to move when I touching it with specific force(not knocking, not punching, just touch and press, speed and accelerate is zero)? $\endgroup$ – Артур Клочко Feb 2 '18 at 20:09
  • $\begingroup$ To the OP: The thing is that some forces have the ability of "adjusting". They can "adjust" to equal and balance out an opposing force. Such "adjusting" forces are for example static friction and the normal force. But they all have a limit. That is what the last graph in the answer here shows. When that limit is overcome, the force gives up and let's go. $\endgroup$ – Steeven Feb 2 '18 at 22:07

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