Net External Torque: Definition & Understanding

[Zynoakib]

My textbook says any torques associated with the internal forces acting in a system of particles are zero and therefore net external torque = total angular momentum/ time

First of all, what is net external torque? Is it just net torque? Is there a thing called internal torque?
Second, why is any torques associated with the internal forces acting in a system of particles are zero?

Thanks!

 

[brainpushups]

First of all, what is net external torque?

The torque due to forces exerted by objects outside of the system boundary.

Is there a thing called internal torque?

Yes. Due to forces exerted by objects inside the system boundary.

Second, why is any torques associated with the internal forces acting in a system of particles are zero?

This is a consequence of Newton's third law because all action-reaction pairs are equal and opposite. The derivation is straightforward and similar to that of the total internal momentum of a system being zero.

Edit: for clarity

 

[Zynoakib]

Thank you!