Force and Newton's 2nd, weird lab results

[Woodpecker]

Homework Statement

Not really a problem, but an analysis question on a lab that I can't figure out. We did an experiment in which we placed a spring scale on a table and attached strings to both sides. The strings went over pulleys and we hung various masses from them. My question is what should the spring scale have read when different masses were hung from each string (ex. 150 grams on one side, 200 on the other)?

Homework Equations

F=ma

The Attempt at a Solution

As far as I can tell, the spring scale should have shown the net force acting upon the whole system. But it only did so inconsistently and with number fudging. Of course, that could have been due to the fact that the force had to be read off a spring scale accelerating away from me at 9.8 m/sec. sq. Overall, not a very well-designed experiment. But anyway, am I correct with the above hypothesis?

 

[qspeechc]

It could be the acceleration of the scale affected the little arm that points to the measured value. It should read the difference between the weights.