Why massless rope is required for making these two forces eq

[Tahmeed]

I am pulling a rope and that rope is pulling another body which is attached to rope. The force applied by me on the rope will be equal to the force applied by rope on that body if 1. The rope can't increase in length due to force, or 2. if the rope is massless.

I understand the reason behind first condition, but why there is that second condition?? what happens if it's massless??

 

[jbriggs444]

The first condition is not correct. You can have unequal forces even if the rope is unstretchable.

Newton's second law applies. f = ma. The net force on the rope is equal to the mass of the rope multiplied by its acceleration. If the forces on the ends are equal and opposite then it follows that either:

1. The rope is massless.
2. The rope has zero acceleration.