volume and surface area

Question

a hollow iron pipe is 21cm long and its external diameter is 8cm. if the thickness of the pipe is 1 cm and iron weigth is 8 g/cm3 then the weigth of the pipe is?

Answer 1

Mass   =    Density * Volume of the hollow cylinder

           =    8 * pi * h * (r₂²  -   r₁²)

           =    8 * (22/7) * 21 (4² - 3²)     [ As outer diameter = 8 cm so radius = 4 cm and as thickness of pipe = 1 cm so inner                                                                 radius = 3 cm ]

           =    8 * 22 * 3 * 7   

           =    3.696 kg