Question

A led pencil consists of a cylindner of wood with a solid cylinder of graphite filled into it. the diameter of pencil is 7mm, the diameter of graphite is 1 mm and the length of the pencil is 10cm. Calculate the weight of the whole pencil, if the specific gravity of the wood is `0.7 (gm)/(cm)^3` and that of graphite is `2.1 (gm)/ (cm)^3`

Answer 1

Volume of graphile=`22/7*(5/100)^2*10`
`=11/140cm^3`
Volume of wood=`22/7*(3.5/10)^2*10-11/140`
`=3.77cm^3`
w=v+d
`=3.77*0.7+(0.078)*2.1`
`=2.8gram`.