# Does standing on bathroom scale depress or compress spring

1. Nov 13, 2012

### Conde

I am unsure how bathroom scales work. Does your weight compress or depress the spring? Do scales measure the upward or downward force? Please explain in detail.

2. Nov 13, 2012

### Staff: Mentor

Welcome to the PF.

http://en.wikipedia.org/wiki/Weighing_scale

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3. Nov 13, 2012

### Conde

Thanks for your reply. I am still confused as to whether when you stand on a scale you are pushing the spring down, which sounds more likely or whether as is written in some books, the spring is pushing up. Also is your weight measured from the scale pushing up, or from your weight pushing down. I appreciate the clarification so I can really understand how the scale works.

4. Nov 13, 2012

### Staff: Mentor

In the linear region of the spring's action, the following equation holds:

F = -kΔx

That means that the force F down on the spring will cause its size to shrink in proportion to the force (and in relation to the "spring constant" k). So if object 2 is twice as heavy as object 1, object 2 will cause twice as big of a Δx shrinkage. The movement down of the scale platform is used to turn the indicator to the corresponding weight of the object. The farther down the spring is compressed, the more the dial moves to show the heavier weight.

http://en.wikipedia.org/wiki/Spring_(device [Broken])

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Last edited by a moderator: May 6, 2017
5. Nov 13, 2012

### Staff: Mentor

Both! You push the spring down and in return, it pushes you back up.

All forces come in similar pairs.

6. Nov 13, 2012

### turbo

Plus, it could be instructive to explore the use of strain-gauges in decent scales. Springs are not the end-all.

7. Nov 13, 2012

### Conde

Thanks everyone,
one last question which I still do not have clarified is whether the scale is measuring the restoring force or the actual force of the weight.

8. Nov 13, 2012

### Staff: Mentor

It must be both or neither: because they come in pairs, it can't be one or the other. But whether it is both or neither is a choice I leave to you. To explain what the scale actually does: the scale has sensors (as turbo suggested) that directly measure the movement (strain) of the spring. The scale then calculates the force required to cause that amount of movement.

9. Nov 13, 2012

### Conde

You have all really clarified this for me. Thanks for your time.