Why Can't a Body Act on Itself?

[Corneo]

I would just like to know why can't a body act on itself with a force. This question arised when I was studying magnetic fields. A wire carrying current sitting in a uniform magnectic friend $\mathbf{B}$ produces a non-uniform magnectic field $\mathbf {B_n}$. Then I want to find the magnectic force, $\mathbf{F_B}$ the wire would experience by $\mathbf {B_n}$. I know this can't happen. A body can't act on itself. The only force this wire can feel is the magnectic force from the uniform field.

A simpler question could be, why can I tuck on my own belt and go up?

 

[Andrew Mason]

I would just like to know why can't a body act on itself with a force. This question arised when I was studying magnetic fields. A wire carrying current sitting in a uniform magnectic friend $\mathbf{B}$ produces a non-uniform magnectic field $\mathbf {B_n}$. Then I want to find the magnectic force, $\mathbf{F_B}$ the wire would experience by $\mathbf {B_n}$. I know this can't happen. A body can't act on itself. The only force this wire can feel is the magnectic force from the uniform field.

A simpler question could be, why can('t) I tuck on my own belt and go up?

The simple answer is found in Newton's laws of motion. The third law: For each action there is an equal and opposite reaction. If a body could apply a net force to itself without pushing against something else, (ie. an action with no reaction) momentum could not be conserved.

In your example of the magnetic field, however, the wire would move.

AM

 

[GSXR750]

The reason a body cannot act on itself is due to the fundamental principles of physics, specifically Newton's third law of motion which states that for every action, there is an equal and opposite reaction. This means that when a body exerts a force on another body, the second body will exert an equal and opposite force on the first body. This is why you cannot pull yourself up by your own belt, as the force you exert on the belt will also be exerted back on you, canceling out any net force.

In the case of the wire in a magnetic field, the wire itself is the source of the magnetic field, so it cannot exert a force on itself. The non-uniform magnetic field it produces will interact with the uniform magnetic field, but not with itself. This is why the wire can only experience a magnetic force from the uniform magnetic field, and not from its own non-uniform field.

Furthermore, in the case of the wire, the force it experiences from the uniform magnetic field is due to the motion of charged particles within the wire, not the wire itself. So, even though the wire is the source of the magnetic field, it is not directly responsible for the force it experiences.

Overall, the principle that a body cannot act on itself is a fundamental concept in physics and is essential for understanding the interactions between different objects in the universe.