My daughter is studying mass, volume and density this week. I created this lab experiment for her class. A Styrofoam cup has a mass of 3.5g and a volume of 60 cm3. If you hold the cup under water and release it, it will float to the surface. What is the minimum number of U.S. pennies needed to put into the cup to keep the cup from floating?
Extension: The class actually has to determine the mass and volume of the cup. How would you find the volume of Styrofoam in a Styrofoam cup?
Source: Original.
( 3.5 + N*3.1) / (60 + N*.36) = 1
Solving for N = (VolumeCup-MassCup)/(MassPenny-VolumePenny) = 20.62
So 21 pennies (dated older than 1982) would sink the cup. For comparison, 27 newer (than 1982) pennies would be needed.
To find the volume of the cup, a few suggestions were made:
1) Perform the experiment with the pennies, then back-calculate the volume
of the cup. (This is probably the most accurate.)
2) Get a graduated cylinder large enough to hold the cup and insert the cup
in until it is just at the level of the water. The water level will
increase equal to the "outside volume" of the cup. Then fill the cup
and measure the "inside volume" of the cup. Subtract to find the cup's
volume.
3) Completely submerge the cup into water in a graduated cylinder and
measure the water level increase. The increase in milliliters is equal
to the cup's mass in cubic centimeters. (This is how the students did
it in the lab.)