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