How Do Spring Scales Measure Weight When Two 100-N Weights Are Attached?

[Francis Oliver]

If two 100-N weights were attached to a horizontal spring scale would the scale read 0, 100, or 200 N, or would it give some other reading?

 

[mathmike]

it would read equal

 

[EnumaElish]

IF IT HAS NEWTONS AS UNITS THEN A horizontal spring scale would read 200 N

Ordinary scales have KGs or LBs. So Newtons need to be converted to KGs or LBs.

 

[Tide]

Please explain how you arrived at 200N.

 

[EnumaElish]

The question doesn't say that the weights are placed at opposite ends. If you place 200N on the same side then that's what you'll "read": 200N.

 

[HallsofIvy]

But it does say that the spring is horizontal! Assuming this is just a simple, graduated, spring that stretches by the gravitational force on a mass, since it is horizontal there is no gravitational force and it will read 0!

 

[EnumaElish]

Do you think it's a "vertical" spring scale being held horizontally? I now realize that when I wrote my first post I replaced "horizontal" with "vertical" in my mind. Then I thought, "what's the point of holding it horizontally, maybe they meant a counter scale (see attached image)." But maybe not?

Site I downloaded the image from. Look under "Commercial" then "Counter scales."

 

[HallsofIvy]

That's a balance scale. It has no springs at all!

I really have no idea what the original problem actually said or intended. I would like to see the exact wording. The original post did specifically say "horizontal spring scale". I can only conclude that it simply isn't going to register any weight at all!

I suppose it could be a spring attached to the wall, set on a horizontal platform but with the weights to be measured hanging vertically. If that's the case, then it would measure 200 N.

Without knowing exactly what the situation is, my best guess based on the wording of the original post is that the spring is sitting on a horizontal platform as are the "weights" and so there is no extension- the spring reads 0.

 

[EnumaElish]

That's a balance scale. It has no springs at all!

I guess that's right. Again I had pictured mentally that it (or some version of it) could have springs under each pan, but apparently not.

Without knowing exactly what the situation is, my best guess based on the wording of the original post is that the spring is sitting on a horizontal platform as are the "weights" and so there is no extension- the spring reads 0.

"I'm with you."

 

[Francis Oliver]

Hello,
Thanks a ton for all of your help. I should have been more specific though because the question is actually based off of a picture. It’s a horizontal spring scale lieing on a table with the weights attached at each end with ropes hanging off the ends of the table so that the scale is in the middle. So with that would it be 200? Really sorry about the confusion.

 

[EnumaElish]

Given this description, my answer would be 200N, or the Kg or Lb equivalent.

 

[HallsofIvy]

Aha! Yes, in that case, the spring would read 200 N. It is exactly the same as if the scale were attached to the ceiling and both weights were attached to the bottom. There would still be 200 N tension in the spring.

 

[mukundpa]

Nothing to say 'Aha'. The two weights are each of 100N and attached on each end creating tension of 100N in each string and hence the reading is 100N.
Just think you are pulling a spring from only one end. It will not get extended. To get the spring stretched we require force on either end of the spring and both are equal. These balancing forces produce elongation in the spring.
In case of vertically suspended spring one force is the weight and the other force is applied by the ceiling, which is equal and opposite. Here the two weights are providing the two forces to elongate the spring and hence the tension is 100N everywhere and the reading will be 100N.

 

[VietDao29]

Nothing to say 'Aha'. The two weights are each of 100N and attached on each end creating tension of 100N in each string and hence the reading is 100N.
Just think you are pulling a spring from only one end. It will not get extended. To get the spring stretched we require force on either end of the spring and both are equal. These balancing forces produce elongation in the spring.
In case of vertically suspended spring one force is the weight and the other force is applied by the ceiling, which is equal and opposite. Here the two weights are providing the two forces to elongate the spring and hence the tension is 100N everywhere and the reading will be 100N.

 

[mukundpa]

please see the attachment
I think this was your question

 

[HallsofIvy]

Nothing to say 'Aha'. The two weights are each of 100N and attached on each end creating tension of 100N in each string and hence the reading is 100N.
Just think you are pulling a spring from only one end. It will not get extended. To get the spring stretched we require force on either end of the spring and both are equal. These balancing forces produce elongation in the spring.
In case of vertically suspended spring one force is the weight and the other force is applied by the ceiling, which is equal and opposite. Here the two weights are providing the two forces to elongate the spring and hence the tension is 100N everywhere and the reading will be 100N.

Yes, how foolish of me!

 

[EnumaElish]

mukundpa, I am sorry I don't "get" it: why would you read 100 and not 200N ? If two persons are tugging at a rope with 100N force each, isn't the tension 200N?

 

[mukundpa]

No, If the tension in the string will be 200N on that end of the string a net force of 100N will act resulting acceleration.
The tension in a string is balancing the force on either side of the system if it is not accelerating.
Please see the attached diagram.

HallsofIve
sorry, I was not meaning that, I only thought that you people are closing the thread.