# Weight of pipe

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• Feb 4th 2011, 07:29 AM
rn5a
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
• Feb 4th 2011, 07:30 AM
Ackbeet
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.
• Feb 4th 2011, 07:47 AM
rn5a
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=105600-59400=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
• Feb 4th 2011, 08:00 AM
Ackbeet
I get essentially what you get. I think the figure of 92.4 kg is incorrect.
• Feb 4th 2011, 08:07 AM
e^(i*pi)
Assuming it's a cylinder then, as for all prisms the volume is cross sectional area multiplied by length or $V = \pi r^2 h = \dfrac{\pi}{4}d^2h$. You may also have a unit mismatch - your length is in metres but the diameter is cm!

$V_1 = \dfrac{\pi}{4} \cdot 8^2 \cdot 2100 = 33600\pi \text{ cm}^3$ -- this is the whole cylinder

Edited this lineBy the same token: $V_2 = 18900\pi \text{ cm}^3$ (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 $\Delta V = V_1 - V_2 = 33600\pi - 18900\pi = 14700\pi \text{ cm}^3$ -- this is the volume taken up by the tube itself

$m = \rho V = 8 \cdot 10^{-3} \cdot 14700\pi = 117.6\pi \approx 369.45 \text{kg , (3sf)}$

The weight (your question erroneously uses kg to describe weight so they might mean find the mass) is given by $W = mg = 117.6g \pi \text{ N}$

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
• Feb 4th 2011, 08:25 AM
Ackbeet
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.