Feeling The Tension
When a 150-gram mass is supported from a 300-gram spring scale, the "weight" of the 150-gram mass is measured as 150 grams. The units of these scales are unusual for a physics discussion, but shucks, that’s the way they work. This is seen in the photo below.
Now suppose that an identical 150-gram mass is supported from strings passing over pulleys, with an identical scale and a spring connected to the strings between the pulleys. This situation is seen in the photo below.
Notice that the face of the spring scale has been purposely positioned so that you cannot see it in the photograph. The question is what that scale will read. It might read zero, because the spring absorbs all of the force when it is elongated. On the other hand, it might read 300 grams, because each weight pulls with a force of 150 grams, but from opposite directions. Or it might read 150, because although each weight pulls with150 grams, for a total of 300 grams, only 150 grams of this force is absorbed by the spring.
In the configuration in the photograph at the left above, which of the following is the correct reading on the scale?
- 0
- 150 grams
- 300 grams
Solution
Reveal

Solution
The answer is that the reading of the scale will be 150 grams, as seen by looking very carefully at the scales shown in the photograph below.
When the mass hangs on the spring scale it stretches the spring so that it reads 150 grams. The spring pulls back with the same 150-gram force. The top end of the spring scale must be supported by that same 150-gram force (plus the weight of the scale). In the situation here, the force that pulls back is the weight of the mass on the opposite end of the setup. It does not matter that the spring is there; according to the law of action and reaction the force acting on each end of the spring must be the same.