A lead pencil consists of a cylinder of wood with a solid cylinder of graphite filled in the interior. The diameter of the pencil is 7 mm, and the diameter of the graphite is 1 mm. If the length of the pencil is 14 cm, find the volume of the wood and that of the graphite.
Concept Used: Volume of the wood = Volume of a pencil – volume graphite
Volume of Cylinder = πr2h
Where r = radius of the cylinder and h = height of the cylinder
Given: Diameter of pencil = 7 mm
Diameter of graphite = 1 mm
Length of the pencil = 14 cm
Assumption: Let r1 be the radius of pencil, r2 be the radius of graphite and h be the height of pencil.
Explanation:
Radius 
Radius (pencil), r1
mm = 0.35 cm
Radius (graphite), r2
mm = 0.05 cm
Height (Pencil), h = 14 cm
Volume of wood in pencil = π (r12 – r22) h
= 
[(0.35)2 – (0.05)2 × 14]
= 
(0.1225 – 0.0025) × 14
= 44 × 0.12
= 5.28 cm3
Volume of graphite = πr22h
= 
× (0.05)2 × 14
= 44 × 0.0025
= 0.11 cm3
Hence, The volume of wood in pencil is 5.28 cm3 and volume of graphite in pencil is 0.11 cm3.