Area Density of Steel Discs

1. Feb 27, 2013

bpinto

1. The problem statement, all variables and given/known data

In general, the density of a three dimensional object is expressed as mass per unit volume. However where an object is of uniform thickness its depth can be combined with the density factor allowing the density to be expressed in terms of mass per unit area. A pair of steel disks joined together are of uniform thickness and each has an area density of 10 grams/cm2

2. Relevant equations

ρ = m/A

3. The attempt at a solution

Mass of bigger disc
10 = m/25∏

Mass of smaller disc
10 = m/4∏

Total mass = 290∏

I don't have any idea what I am doing wrong as the answer is suppose to be 58∏

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Last edited: Feb 27, 2013
2. Feb 27, 2013

SteamKing

Staff Emeritus
Your calculation of the mass seems to be correct. Is your problem statement complete?

3. Feb 27, 2013

bpinto

Sorry it is incomplete I thought it was part of the attachment, the actual question asks:

What is the mass of the whole object?

4. Feb 27, 2013

CWatters

I agree. Your calculation for the total mass looks right but was that the question?

You wrote...

That's correct but to me it's a slightly a strange way to do it. I would write:

Mass = area * area density

= 25∏ * 10
= 250∏

5. Feb 27, 2013

haruspex

Looks like it was supposed to be 2g/cm2

6. Feb 28, 2013

bpinto

thanks for the help guys