Posted by victor on Monday, July 18, 2011 at 11:16pm.
Pelicans tuck their wings and freefall straight down when diving for fish. Suppose a pelican starts its dive from a height of 15.0 m and cannot change its path once committed.
If it takes a fish 0.25 s to perform evasive action, at what minimum height must it spot the pelican to escape? Assume the fish is at the surface of the water.

physics  drwls, Tuesday, July 19, 2011 at 2:36am
The fish must spot the pelican when it is 0.25 s (or more) from hitting the water.
The pelican takes
t = sqrt(2H/g) = 1.75 s to hit the water if it starts its dive at H = 15 m.
Its vertical drop 1.50 s after starting the dive is y = (1/2)g*(1.50)^2 = 11.03 m
It is 15  11.03 = 4.97 m above the water surface at that time,
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