falling objects question?

Question

A large piece of wood, of mass 80 kg, falls over a waterfall and into the drop pool below. It enters the pool travelling at 20 ms-1 and is then subject to an upwards acceleration of 12 ms-2 due to its buoyancy. How long does it take before it bobs to the surface of the pool?

Answer 1

I wonder that myself...

The force that acts on the block once it hits the water is the same force that returns it to the surface. Assuming that this is a point pass
F=B - W
F- resulting force
B - buoyant force
W - weight of the object.

so acceleration when it hits the water is the same as the object breaks the surface coming up a= 12m/s^2 since a=F/m

V=V0 - at since V=0 just before the block begins its ascend we have

V0=at
t= V0/a= 20/12=1.7 sec

Since a=a1=a2 then the total time is
t-total = 2t= 3.4 sec.

Note: these is an approximation since the geometry of this piece of wood upon contact with water will not behave as a point mass assumed and the analysis will much more complex.