Answer
Use Newton's 2nd and 3rd laws of motion.
Work Step by Step
Recall Newton’s second law of motion given by the following equation
$$\sum F=ma$$
This then implies that the objects which are connected to opposite ends via cord, rope or a spring of a negligible mass should exert a negligible net force. Otherwise, the Newton’s second law would lead to an infinite acceleration.
Finally, by the third law of motion, we know that that the forces acting on each end of the cord are equal in magnitude but opposite in direction while the magnitude of the forces exerted on the objects are the same.
