# Homework Help: This is supposed to be simple

1. Jun 6, 2010

### derivethis

Hi all! This is my first post here. I have a question regarding how to calculate the mass density of a steel sphere. Any help is much appreciated!

1. The problem statement, all variables and given/known data
The mass of the sphere is equal to 8.4 grams. The volume of the sphere is equal to 1060 cm^3.

2. Relevant equations
The mass was found during our lab experiment. The volume was calculated from the equation: V = (4/3) pi R^3. The diameter of the sphere was 12.65 cm. Therefore, I calculated the volume as follows: (4/3) pi (12.65/2)^3 = 1060 cm^3.

3. The attempt at a solution
Mass density is equal to mass/volume; therefore it should be 8.4g/1060cm^3 = 0.0079gm/cm^3. However, I'm also supposed to calculate the percent error for my lab report, and the "true" value for the mass density of steel is given as 7.9 gm/cm^3. Percent error is equal to (|Accepted - Measured|)/Accepted * 100, which, in this case, would be (|7.9 - 0.0079|)/7.9 * 100 = 99.9%. This gives me a percent error of almost 100%! What am I doing wrong in my calculations? Please help! Thank you!!

2. Jun 6, 2010

### l'Hôpital

Are you sure you have your units right?

1 gram = 10^-3 kg. And 1 cm^3 = 10^-6 m^3.

so 1 g/cm^3 = 10^-3 / 10^-6 kg/m^3 = 10^3 kg/cm

so your 0.0079 g/cm^3 = 7.9 kg/m^3.

3. Jun 6, 2010

### vela

Staff Emeritus
Your volume is about 1000 cm3, which is 1 liter. Does that sound right?

4. Jun 6, 2010