# Mass of disk given mass per unit

1. Apr 12, 2013

### aaronfue

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

I need to find the mass of a disk with a hole, given inner and outer diameters of 0.10m and 0.30m, respectively. The disk has a mass per unit of 20 $\frac{kg}{m^2}$.

3. The attempt at a solution

Since I needed mass units, I thought that I could multiply the mass per unit by the area of each disk, the disk as a whole and the missing hole. (The missing will be subtracted from the whole)

So,

Whole disk:
20 $\frac{kg}{m^2}$ × $\pi$ × 0.30 m2 = 5.65 kg

Missing hole:
20 $\frac{kg}{m^2}$ × $\pi$ × 0.10 m2 =0.628 kg

Total mass of disk:
5.65 kg - 0.628kg = 5.03 kg

Is this correct?

I will also have another question that I will need answered related to this but I will try it on my own and provide my calculations.

Thanks for the help!

2. Apr 12, 2013

### SteamKing

Staff Emeritus
What is the formula for the area of a circle?

3. Apr 13, 2013

### aaronfue

Area of Circle: $\pi$r2

4. Apr 13, 2013

### SteamKing

Staff Emeritus
And have you used the correct r in your calculations? Hint: Read the OP again very carefully.

5. Apr 13, 2013

### aaronfue

Oh......I used the diameter instead of the radius!!

20$\frac{kg}{m^2}$ × 0.152 = 1.414 kg

20$\frac{kg}{m^2}$ × 0.052 = 0.158 kg

Total mass: 1.414 - 0.158 = 1.256 kg

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