1. How many pencils would it take to build a one-man raft?
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How many pencils would it take to build a one-man raft?
Pencils are usually made of a combination of cedarwood and a graphite/clay mixture, with a combined density of 0.6g/cm3
The average person weighs about 70kg.
Volume of a pencil: 16 x pi x 0.4^2 = 0.64 pi cubic centimetres. Weight = 0.6 x 0.64 pi grams. Buoyancy = 0.4 x 0.64 pi grams = pi grams. To hold up 70,000 grams, 70,000 / pi = 87,038 pencils.
Pencils are roughly cylindrical, about 16cm long and 8mm across.

2. The Answer 145,000 pencils per person ≈ £14,500 ≈ 3 Jet-Skis
Density = 0.6g/cm3
Volume = 1.6 x π x 0.42
≈ 0.80cm3
70,000 ÷ 0.48
≈ 145,000 pencils
Volume of a pencil: 16 x pi x 0.4^2 = 0.64 pi cubic centimetres. Weight = 0.6 x 0.64 pi grams. Buoyancy = 0.4 x 0.64 pi grams = pi grams. To hold up 70,000 grams, 70,000 / pi = 87,038 pencils.
Weight = D x V
= 0.48g