Simple Ratio?

1. Jan 11, 2007

Mesmer

A concrete column has a diameter of 350mm and a length of 2m. If the density (mass/volume) of concrete is $$2.45\frac{Mg}{m^3}$$. determine the weight of the column in pounds

The answer to this problem is given as 1.04 kip not pounds like the question ask for, but that is not my problem. This seems like and probably is an easy problem to answer but I cant figure out what I'm doing wrong. Here is what I've done to solve this problem:

The first thing I did was change the Mg to Kg and the mm to m. No biggy.
The only other info I have is the dimension of the column. I used that to find the volume of the column $$V_c= \pi*r^2*h$$. Then i just formed a simple ratio with the mass as my unknown. This method does not work for me. any Ideas.

2. Jan 11, 2007

cristo

Staff Emeritus
Did you remember to convert your mass into pounds?

Post your full solution if you're still having problems, as that's the only way we'll be able to see where you're going wrong!

edit: Also, did you note that mg≠Mg?

Last edited: Jan 11, 2007
3. Jan 11, 2007

Mesmer

I did convert the mass to weight using Weight = (mass)(gravity)

height=2m
density=$$2450\frac{kg} {m^3}$$

Now all I have to do Is set up the ratio :$$2450\frac{kg}{m^3}=\frac{?}{0.141372m^3}$$

Then I solved for the unknown (?) and converted that answer to lbs

Last edited: Jan 11, 2007
4. Jan 11, 2007

cristo

Staff Emeritus

Pounds are units of mass. There is a mistake in your question.

OK

I would write this as $$\rho=\frac{m}{V} \Rightarrow m=\rho V$$, where $\rho$ denotes density, m denotes mass, and V denotes volume.

Rearranging in terms of algebraic symbols is easier. Unfortunately, I get a different value for V than you. Show your calculation.

Ok, this should work, show your working!

Last edited: Jan 11, 2007
5. Jan 11, 2007

Kurdt

Staff Emeritus
Where does the radius of 150m come from? A millimeter (mm) is one thousandth of a meter.

Also your method is wrong, do as cristo suggests and find the mass of the column then convert to pounds.

6. Jan 11, 2007

Mesmer

I see what I did now. When I found the radius I used 0.150m instead of the correct 0.175m. Now I have it right :) Thank you.

7. Jan 11, 2007

cristo

Staff Emeritus
You're welcome, but please take onboard my point about algebraic expressions; it will help a lot in future study!