Question

Difficulty: MediumMeasurement of Mass and Weight

A spring balance is calibrated in kilograms at sea level where g=10.0 m s2g = 10.0\text{ m s}^{-2}. An object measured with an equal-arm beam balance at sea level is found to have a mass of 36.0 kg36.0\text{ kg}. If this object is taken to a high altitude where g=9.0 m s2g = 9.0\text{ m s}^{-2} and suspended from the same spring balance, what reading will the scale of the spring balance display?

  1. 32.4 kg32.4\text{ kg}Answer
  2. B
    36.0 kg36.0\text{ kg}
  3. C
    40.0 kg40.0\text{ kg}
  4. D
    324.0 kg324.0\text{ kg}

Answer

The spring balance scale will display a reading of 32.4 kg32.4\text{ kg}.
An equal-arm beam balance measures true invariant mass (36.0 kg36.0\text{ kg}) by comparing gravitational moments on two arms. A spring balance measures force (weight). At high altitude, the gravitational force acting on the mass is W=36.0 kg×9.0 m s2=324 NW = 36.0\text{ kg} \times 9.0\text{ m s}^{-2} = 324\text{ N}. Because the spring balance scale was calibrated assuming sea-level gravity (10.0 m s210.0\text{ m s}^{-2}), it converts force to indicated mass as 324 N/10.0 m s2=32.4 kg324\text{ N} / 10.0\text{ m s}^{-2} = 32.4\text{ kg}.

Step-by-Step Solution

1
Calculate the true weight of the object at the new altitude
W=m×galtitude=36.0 kg×9.0 m s2=324 NW = m \times g_{\text{altitude}} = 36.0\text{ kg} \times 9.0\text{ m s}^{-2} = 324\text{ N}
Weight is the force of gravity acting on a mass at a specific location.
2
Determine the indicated mass reading on the calibrated spring balance scale
\text{Reading} = \frac{W}{g_{\text{calibration}}} = \frac{324\text{ N}}{10.0\text{ m s}^{-2}} = 32.4\text{ kg}
The spring balance measures tension force but its scale was calibrated using sea-level gravity (10.0 m s210.0\text{ m s}^{-2}) to indicate mass.

Key Concept

Mass is an intrinsic property measured independently of gravity by a beam balance, whereas weight is a force measured by a spring balance, causing mass-calibrated spring balances to give different readings under varying local gravitational accelerations.
Estimated Time:1m 0s
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