Physics
posted by Alex .
Buoyancy.
A Team of 6 scouts plan to cross a lake on a raft they designed.
The dimensions of each plank is 30 cm x 30 cm x 3m.
The density of the wooden planks is of .80.
The average mass of the members is 65KG. And for their safety they want the top of the raft to be 3 cm above the surface of the water. How many wooden planks do they need to complete their project?
So far i found
V1 = .248
V = .27
P1 = .8
P = .73
using V1/V = P/P1.
Total mass of cargo ( scouts ) 6 x 65 = 390 KG. I really don't know where to go from here?!?!

Add the mass of scouts and N planks.
That must equal the mass of displaced water.
Plank mass (each) = 0.80*0.3*0.3*3*1000 kg/m^3
= 216 kg
Displaced water mass per plank:
= 0.3*0.27*3*1000 kg/m^3
= 243 kg
390 + 216 N = 243 N
N = 14.4 planks