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Aptitude - Volume and Surface Area - Discussion

Discussion Forum : Volume and Surface Area - General Questions (Q.No. 5)
Volume and Surface Area
5.
A hollow iron pipe is 21 cm long and its external diameter is 8 cm. If the thickness of the pipe is 1 cm and iron weighs 8 g/cm3, then the weight of the pipe is:
3.6 kg
3.696 kg
36 kg
36.9 kg
Answer: Option
Explanation:

External radius = 4 cm,

Internal radius = 3 cm.

Volume of iron
= 22 x [(4)2 - (3)2] x 21 cm3
7
= 22 x 7 x 1 x 21 cm3
7
= 462 cm3.

Weight of iron = (462 x 8) gm = 3696 gm = 3.696 kg.

Discussion:
60 comments Page 2 of 6.
Neelam shelke said:   1 decade ago
Thickness of the pipe is 1cm, and external radius is 4cm, so 4-1=3, thus the internal radius is 3, as per my thinking.
Mobi001 said:   1 decade ago
If the thickness of pipe is 1 cm the internal diameter should not be 7 cm if yes then internal radius must be 3.5 ?
Preeti said:   1 decade ago
Can some one explain how thickness and external radius are related. I understood we have to subtract but why?
Praveen Singh said:   1 decade ago
Akanksha External Radius=4 and in problem thickness=1, then Internal radius=4-1=3.thats hit...
DR Sharma said:   1 decade ago
Internal radius is subtracted from the external radius to find out the volume of hollow pipe.
CHINMAYE said:   7 years ago
@Rohan.

It's given in the question that iron weighs 8 g/cm3. It's nothing but a density.
Mobi001 said:   1 decade ago
Thats fine. I understood the logic.

Because from both side the 1cm will be deducted.
Venki said:   1 decade ago
Yes its a cylinder but it has 1 mm wall thickness. So we calculate the weight of iron.
Sahi said:   1 decade ago
@Shivani.

22/7 is the value of pi. Used in calculating volume of cylinder = pi*r^2*h.
Rajadurai said:   9 years ago
Why [4]^2 - [3]^2 while calculating volume?

Anyone please explain this step.
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