Question

What is the smallest amount of logs (ρ = 725kg/m3) with radius 0.08m and length 3.0m that can be used to build a raft that will carry 4 people with mass of 80kg each?

Answer 1

Weight of logs = mass* gravity

Weight of logs = density*volume*gravity*n

Where n is number of logs we need to find.

Weight of logs = 725*pi*0.08*0.08*3*9.81*n

Weight of logs = 428.78*n

Weight of four people = 80*9.81*4

Weight of four people = 3139.2

Weight of water displaced = n*pi*0.08*0.08*3*9.81*1000

Weight of water displaced = 591.425*n

Now equate

591.425*n = 3139.2 + 428.78*n

162.645*n = 3139.2

n = 19.3

As logs cannot be in decimals, so number of logs should be 19