What do scales actually tell us?

  • Context: Undergrad 
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Discussion Overview

The discussion centers around the nature of measurements provided by scales, specifically whether they indicate weight (force) or mass. Participants explore the implications of gravitational acceleration on these measurements and the conventions used in different unit systems.

Discussion Character

  • Conceptual clarification, Technical explanation, Debate/contested

Main Points Raised

  • One participant questions whether a scale's reading of weight (e.g., 98kg) actually reflects mass when considering gravitational acceleration, suggesting that scales measure force rather than mass.
  • Another participant asserts that scales measure weight (force) in the gravitational MKS system, where the kilogram-force (Kgf) is a fundamental unit of force, and mass is derived from it.
  • A different viewpoint states that a scale measures the upward normal force it exerts, and that for psychological reasons, this force is often presented in kilograms by dividing by the acceleration due to gravity (9.8 m/s²), which could lead to confusion regarding mass and weight.
  • A later reply acknowledges the division by 9.8 m/s² as mentioned by another participant.

Areas of Agreement / Disagreement

Participants express differing views on whether scales provide a measure of weight or mass, and the discussion remains unresolved with multiple competing perspectives on the interpretation of scale readings.

Contextual Notes

There are assumptions regarding the gravitational context (e.g., Earth) and the psychological implications of how weight is presented, which may not be universally applicable.

Pengwuino
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If you stand on a scale that says you're weight is 98kg, does that mean the scale is tellign you you weigh 98kg * m/sec^2? which inevitably means your mass is 10kg? (ok not using real world numbers here). It seems like scales weigh Force because in an elevator going down, your weight changes if your under an acceleration (and if you have ever put your arm on something while being weighed, you know your weight decreases).

So actually, what I am asking is if scale's #'s are fixed (divided by 9.8) to show mass instead of your weight
 
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Actually it is measuring weight(force) in the gravitational MKS system. In this unit system the Kgf is the unit of force and is considered fundamental and mass is derived.
 
A scale measures the magnitude of the upward normal force it applies on you.
In the english system, this force is measured in pounds (lbs).
In the metric system, this force is measured in Newtons (N). However, possibly for psychological reasons, that force in Newtons is divided by 9.8m/s^2 and presented to you in kilograms (which would be your mass, as long as you use this scale on earth).
 
So it is divided by 9.8... thanks robphy.
 

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