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What does the scale reading represent?

  1. Mar 18, 2012 #1
    Say that an object is placed on a scale and it gives a reading. Does that number represent the object's weight (w = mg) or mass (just m)? I'm a bit confused because my physics teacher told me that it is measuring its mass...is this correct?

    Many thanks.
  2. jcsd
  3. Mar 18, 2012 #2

    Doc Al

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    Depends what kind of scale it is. If it's a scale like a bathroom scale, it essentially gives you a measure of the force pressing against it. The reading itself could be a weight or a mass, since they are proportional. But use that scale on the surface of the moon and it will read less, since your weight would be less there. But your mass hasn't changed.
  4. Mar 18, 2012 #3


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    Most 'weighing machines' measure a force - due to a mass in the presence of a gravitational field. If you use a balance (see-saw) you are comparing the weight forces of two masses so you can eliminate any variation of g. If you use a spring balance, you are just measuring the weight force in the presence of an uncertain value of g.

    However, you can measure the mass of an object in the absence of any g field or a table to place it on if you include the object in an oscillating arrangement of springs and known masses. The mass of the object can then be found by measuring the frequency of oscillation of the system. Of course, this would involve knowing the values of the other masses and the exact stiffnesses of the springs - but the method would work anywhere.
  5. Mar 18, 2012 #4
    The scales measure the objects weight, because when you place the object on the scales it is influenced by the gravitational pull. Weight=mg therefore to get the mass, you must divide 9.81
  6. Mar 19, 2012 #5


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    Does the scale have a balance beam and a counterweight? Then it measures mass. If it doesn't, then it probably measures weight, which is a force.
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