How to Get Weight From Specific Gravity

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“Weight” refers to the force of gravity on an object. It’s generally determined by multiplying the mass of an object by the acceleration of gravity (which on earth is 9.81 meters per second squared, or 32.174 feet per second squared). “Specific Gravity” relates to density. It’s the ratio of an object's weight relative to the equal weight of a standard (the standard generally used is water at four degrees Celsius). To calculate an object’s weight from its specific gravity you have to figure out the object’s specific weight and multiply this result by the object’s volume.

Things You'll Need

  • Calculator
  • Pencil
  • Paper
  • Determine the density of your object. Start by understanding how specific gravity is determined. Specific Gravity equals the Density of a Substance divided by a standard (usually the Density of Water at four degrees Celsius). But where density is measured in units specifying mass and volume--such as kilogram per cubic meters (kg/m3)--specific gravity is a unitless number. To determine density from specific gravity you simply reverse the operation. You multiply the Density of Water at four degrees Celsius by the Specific Gravity. And since density is measured in units, the density of your object takes the same unit of measurement you use for specifying the density of water. Pure water at four degrees Celsius has a density of 1000 kg/m3. So if, for example, an object has a specific gravity of one, its density is then calculated as 1000 kg/m3 because one multiplied by 1000 is 1000.

  • Calculate the specific weight of your object. Specific weight refers to the weight of an object per unit volume. The formula for calculating specific weight is Density multiplied by the Acceleration of Gravity. The acceleration of gravity is 9.81 meters per second squared (m/s2) or 32.2 feet per second squared (ft/s2). So if, for example, an object has a density of 1000 kg/m3, its specific weight is calculated as 1000 multiplied by 9.81. This equals a specific weight of 9810 Newton per meter cubed (N/m3). In kilo-Newtons this is equal to 9.81 kN/m3.

  • (Optional) Convert the kilo-Newton/meters cubed (kN/m3) units of measurement to the imperial pound/feet cubed units of measurement (lb/ft3). This is accomplished by multiplying the kN/m3 value by 6.36590. So if, for example, an object has a specific weight of 9.81 kN/m3, when 9.81 is multiplied by 6.36590 you see that 9.81 kN/m3 is the equivalent of approximately 62.4 lb/ft3.

  • Determine the volume of your object. To calculate the volume of a solid object by water displacement, subtract your starting volume of water from the volume of water when your object is submerged in the water. To determine the volume of your object based on its shape you can use any of the following formulas or combination of formulas (for objects with a complex form):

    Rectangle / Square / Cube: Length multiplied by Width multiplied by Height
    Pyramid: Length multiplied by Height divided by three.
    Cylinder: 3.14 (pi (?)) multiplied by the square of the base’s Radius. The result multiplied by the Height of the cylinder.
    Cone: 3.14 (pi (?)) multiplied by the square of the base’s Radius. The result multiplied by the Height of the cone. Divide by 3.

    Use the “imperial unit” measuring system if you converted to pound in feet in step three. Use the metric “SI unit” measuring system if you choose not to convert to pounds in feet in Step 3.

  • Multiple the specific weight of your object by the volume of your object to calculate its weight.

References

  • Photo Credit laboratory scale weighbeam image by Pali A from Fotolia.com
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