
Weight of pipe
Can someone please help me with the following problem?
An iron pipe is 21 m long & its exterior diameter is 8 cm. If the thickness of the pipe is 1 cm & iron weight 8 g/cu.cm, find the weight of the pipe.
How do I do this? I have found the volume of the pipe by first finding the external volume, then the internal volume & then finding the difference but what do I do after that to find the weight?
Thanks,
Ron

Just multiply by the given density of iron. Note that you're asked to find the weight, not the mass. Thus, you'll need also to multiply by g.

I tried what you suggested but am not getting the correct answer. This is what I did:
External Volume=22*4*4*2100/7=105600 cu.cm
Internal Volume=22*3*3*2100/7=59400 cu.cm
Volume=10560059400=46200 cu.cm
Weight (or mass...whatever it is)=46200*8=369600 gm=369.6 kg
But the answer is 92.4 kg.
Thanks,
Ron

I get essentially what you get. I think the figure of 92.4 kg is incorrect.

Assuming it's a cylinder then, as for all prisms the volume is cross sectional area multiplied by length or $\displaystyle V = \pi r^2 h = \dfrac{\pi}{4}d^2h$. You may also have a unit mismatch  your length is in metres but the diameter is cm!
$\displaystyle V_1 = \dfrac{\pi}{4} \cdot 8^2 \cdot 2100 = 33600\pi \text{ cm}^3$  this is the whole cylinder
Edited this lineBy the same token: $\displaystyle V_2 = 18900\pi \text{ cm}^3$ (you should see if you can figure out where this figure came from)  this is the volume of empty air on the insde
Edited this lineHence the difference in volumes $\displaystyle \Delta V = V_1  V_2 = 33600\pi  18900\pi = 14700\pi \text{ cm}^3$  this is the volume taken up by the tube itself
$\displaystyle m = \rho V = 8 \cdot 10^{3} \cdot 14700\pi = 117.6\pi \approx 369.45 \text{kg , (3sf)}$
The weight (your question erroneously uses kg to describe weight so they might mean find the mass) is given by $\displaystyle W = mg = 117.6g \pi \text{ N}$
I don't get your answer but I can't see where I went wrong.
Edited  thanks to Ackbeet pointing out where I went wrong

e^(i pi):
The thickness of the pipe is 1 cm. You used 7 as the inner diameter, but the thickness has to be subtracted twice from the outer diameter in order to get the inner diameter.