Question

A raft of wood (density $600kg/m^3$) of $120kg$ floats in water. How much weight can be put on the raft to make it "just sink"?

• A. $40kg$
• B. $80kg$
• C. $120kg$
• D. $200kg$

Answer 1

The correct option is B $80kg$

Given, mass of raft $=120kg$
Density of wood $=600kg/m^3$
Volume of raft $\frac{12}{600}=\frac{1}{5}{m}^3$
Upthrust on raft when just immersed
$=\left(\frac{1}{5}\right)\times 100=200kgf$
Let the additional weight placed on raft $=M$ $kg$ $f$
When the raft neither sinks not floats, the weight of raft ling with additional weight is equal to upthrust
$120+M=200$
$\Rightarrow$ $M=200-120=80kg$