ISOSTATIC EQUILIBRIUM Pre-Lab Exercise

PART 1: VOLUME, MASS, & DENSITY

Read the information below to answer the pre-lab questions.

Density is a measure of mass per unit volume. To use water as an example, a gallon (a unit of volume) weighs about 8.33 pounds (a unit of mass). Therefore, the density of water is 8.33 pounds per gallon. We can use any measurement of mass and/or volume to express density. Water’s density is also 62.4 pounds per cubic foot (62.4 lbs/ft³), 1.0 kilogram per liter (1.0 kg/L), or 1.0 gram per cubic centimeter (1.0 g/cm³), which is the same thing as 1.0 gram per milliliter (1.0 g/mL).

In this lab exercise, we will use the standard Metric System unit for density, which is grams per cubic centimeter: g/cm³

To measure the density of something in g/cm³, we need to measure both its mass in grams and its volume in cubic centimeters. Density is always equal to mass divided by volume, not the other way around. A common error students make is to calculate volume divided by mass, which is the inverse of density.

Measuring mass is easy; you can just weigh an object on a scale. When you stand on a bathroom scale, you probably measure your mass (weight) in pounds. But you could just as accurately express it in grams, although it will be a large number: mass in pounds x 454 = mass in grams.

Measuring volume is more involved. In the lab, you will measure volume in two ways: by linear dimensions and by water displacement.

Volume from linear dimensions:

Measuring volume with linear dimensions works for any object that is geometrical: cubes, spheres, cylinders, cones, and so forth. For example, if you have a rectilinear object—like a brick or a rectangular block of wood—you can measure its volume by measuring its length, width, and thickness and multiplying the three together. For example, this object has a volume of 25cm x 9cm x 8cm = 1800 cm³

Volume from water displacement:

If you want to know the volume of an object that has an irregular shape—like your body, or a piece of rock—then the easiest method is water displacement. An object immersed in water will displace a volume equal to the object’s volume. For example, if you filled a bathtub precisely to the brim and were then able to catch all the water that overflowed when you immersed yourself, the volume of the displaced water would be equal to your body volume.

In the lab, we will measure the volume of rock samples by measuring the amount of water they displace in a graduated cylinder, like this:

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PART 2: ISOSTATIC EQUILIBRIUM DEMONSTRATION

Watch and listen to the short video below to answer the pre-lab questions for this section.

Note: You can turn CAPTIONS on or off by clicking “CC” at the bottom right of the video screens. Also, you can view the videos at higher resolution by choosing a better quality option from the menu at the bottom right of the screen.

Video by Keith Meldahl