Science Buddies: "Ask an Expert"

Hi. I just completed the Science Buddies project on buoyancy. I made my own boats out of aluminum foil and filled them with pennies until they sank. I calculated the volume of each boat using rice. I determined the mass of the pennies. I used the formula density= mass/volume to calculate the density. The density of all five of my boats came out to be less than 1. I know the density of water is 1 gram per cubic centimeter. I thought than an object will only sink if the density of the object is greater than the density of water. I am trying to write up my conclusion and analysis but I don't really understand how my results explain why the filled boats sank. Am I calculating the density wrong?

Here is an example of my work:

Boat 1
Volume 671 mL
Weight/Mass of Pennies 95 grams
Calculated density rounded to the nearest hundredth: 0.14 grams per mL

Please help! I do not want to finish typing up my explanations until I know my logic and my math is correct.

Thank You!

Hi cmm063001,

You are correct that an object will sink if its density is greater than the density of water. The volume and mass data you posted does give a density of 0.14 g/mL. So your calculations are correct.

You should include the mass of the boat in the density calculation. For example, say that the mass of the boat is 600 g. The total mass (boat and pennies) is 695 g. And the density is 695/671 = 1.04 g/mL.

I hope this helps. Please ask again if you have more questions.

A. Norman

Thank you so much! That makes total sense.