Discussion
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 weight 8 g/cm3, then the weight of pipe is?
Answer: C
The pipe weighs 3.696 kg.
The external radius is 4 cm and, with a 1 cm wall, the internal radius is 3 cm. The volume of iron is the difference of the two cylinders:
\[ \begin{aligned} V &= \pi(4^{2} - 3^{2}) \times 21 \\ &= \tfrac{22}{7} \times 7 \times 21 = 462\ \text{cm}^{3} \\ \text{mass} &= 462 \times 8 = 3696\ \text{g} \end{aligned} \]
3696 g is 3.696 kg. 36.96 kg misplaces the decimal point. A solid rod of the same size would weigh \(\tfrac{22}{7} \times 16 \times 21 \times 8\) g, about 8.4 kg, so forgetting the hollow core does not give the other options either.
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