A rock is dropped from a tree of height 25 m into a lake
(depth 5.0 m) below. After entering the water, the rock then
fl oats gently down through the water at a speed of 1.5 m/s to
the bottom of the lake. What is the total elapsed time?
mgh=mv²/2
v=sqrt(2gh) = sqrt(2•9.8•25) =22.14 m/s.
t1=v/g = 22.14/9.8 = 2.26 s.
t2=d/v2= 5/1.5 = 3.33 s.
t=t1+t2 =2.26+3.33 = 5.59 s.