Physics
posted by Ellen .
A hollow plastic spherical bobber of mass 0.005 kg and volume of 50 cm3 floats on the surface of a lake supporting a submersed fishhook and a bait with a combined mass of 0.01 kg and a combined volume of 2 cm3. The density of water is 0.001 kg/cm3.
If a fish grabs the bait, find the minimum downward force, Ffish, it must exert to pull the bobber completely under the surface of the water. You may assume that the apparent weight of the fish is zero, and the fishing line is completely slack.

So you want the bouyant force of the submerged bob.
Mass: .005+.01 kg
Volume: 50cm^2+2cm^2
mass of displaced water: densitywater*volume=1g/cm^3*52cm^2=52g
bouyant force= 9.8N/Kg * (.052kg)
net force=9.8(5215)N
check my thinking. 
thank you so so so much. the answer is .3626

The force that is

The force that is exerted by the bobber and the hook and bait will be equal to the combined mass and the combined volume (0.015 kg and 52 cm^3) not sure why though.
The buoyant force if the object was completely submerged would be the density of water*V displaced*g (.001*52*9.8).
The difference between these two (0.3626) would be the force the fish would need to exert to completely submerge the bobber.