Newton’s First Law Describes Inertia
What is Inertia?
Inertia is the resistance of an object with balanced forces acting on it to change its state of motion.
In the diagram to the right, the stick figures are pulling with equal force in opposite directions on a rope.
The forces acting on the rope are balanced, thus considering only the two balanced forces, the rope remains at rest (stationary).
In the diagram below, the two stick figures are walking at the same speed. At the same time, the figures are pulling with equal force in 
opposite directions on a rope.
The forces on the rope are balanced, thus the balanced forces do not change the ropes state of motion. In other words, the rope remains taught between the stick figures and the rope continues to move at the same speed as the stick figures.