How is a reading on a spring scale equal to a friction force?
A scale is a force indicator. The reading on a scale is simply telling you what force is being applied. If you try dragging an object across the floor using a spring scale, you are applying a force to the spring scale which is applying a force to the object. The spring scale cannot put more force on the object than you are putting on the scale, and it has to put all of the force that you put on it on the object you are pulling. The only other force on the object besides your pulling force is friction. If the object is traveling at a constant velocity, the forces on it are equal. That means that your pulling force has to equal the force of friction. The scale that you are using will read the amount of force you are putting on the object which will tell you how much friction is pulling backwards on the object because these forces are equal strengths.