physics
posted by maria .
Donald Duck and his nephews manage to sink Uncle Scrooge's yacht (m = 4843 kg), which is made of steel (ρ = 7850 kg/m3). In typical comicbook fashion, they decide to raise the yacht (which is now on the bottom of a lake) by filling it with pingpong balls. A pingpong ball has a mass of 2.70 g and a volume of 3.35ยท105 m3.
How many balls are required to float the ship?

The amount of mass that each ball will lift is (in grams)
Pwater*V  Mball
= 1.00 g/cm^3)*3.35*10^1 cm^3  2.70
= 30.8 g
The number of pingpong balls needed is 4843*10^3g/30.8 = 157,200
The density of steel does not matter. The answer depends upon the total mass that must be lifted.