enter image description here

In the above image I have a block of mass m hanging from a mass less pulley with the help of a massless string. The other end of the string is attached to a wall.

$F_g$ is gravitational force acting downward and $F_s$ is the restoring force of spring acting leftward. T represents tension in the string.

I had to find the reading of the spring balance and assumed that it would be the force on the spring balance(to measure extension of spring).

However I can't seem to figure out the tension force between the spring balance and the wall. If I consider that spring balance was a block, then tension force would be leftward and of magnitude T. But what about the tension force leftward? (on the wall)

Similarly in the following image, I again need to find the weight measured by the spring balance:

enter image description here

My intuitive understanding of the problem is since the bodies are not accelarating, the problem is similar to the previous one and therefore the weight measured by the spring is the weight of one block. But is this correct? if so, what would be the direction of restoring force of the spring?

Also if instead, of the spring balance, I imagined a rigid block at the same position, then the net force on the block would be zero, but when I use the spring balance, the measured weight is different, why is this so?

Sorry if I sound very confused.


I now know that the tension leftward = T,(in the first diagram) but still don't understand which direction the restoring force of the spring would be (in the second diagram), since force is being applied in both directions

  • $\begingroup$ Something to think about: Is the tension $T$ in image 1 the same as the $T$ in image 2? $\endgroup$ Commented Oct 26, 2017 at 10:50
  • $\begingroup$ cool diagrams.. $\endgroup$ Commented Oct 26, 2017 at 12:51

1 Answer 1


What you must remember is that the spring is not moving and so the force on one side of it is the same as the force on the other side of it.

I have combined your two diagrams.

enter image description here

So you either think of the wall as pulling on the string in your first diagram or the left side bit of string in your second diagram.

  • $\begingroup$ Thank you! I did think of it that way but was scared it was wrong. One clarification though: in the first diagram, I know that the restoring force due to spring is leftward (opposite applied force), but what about in the second situation(with two blocks?) $\endgroup$
    – GreenApple
    Commented Oct 26, 2017 at 12:02

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