Problem Set 2.10: | 1
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3 | 4 | 5 | 6
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7 | 8 | 9 | 10
| 11 | 12 | 13 | 14
| 15 | 16 | 17 | 18
| 19 | 20 | 21 | 22
| 23 | 24 | 25 | Go up
- by <your
name>, <year>
1. Pelicans tuck their wings and free fall straight down when diving for fish. Suppose a pelican starts its dive from a height of 16.0 m and cannot change its path once committed. If it takes a fish 0.20 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.
I know I can figure out the time it takes the pelican to reach the water: (s = -16.0 m, u = 0, a = -9.80 m/s/s) using:
s = ut + 1/2at2
-16.0 m = 0 + 1/2(-9.80 m/s/s)t2
t = 1.807 s
I also know that the fish needs to see the pelican exactly .20 seconds before this or 1.807 s - .20 s = 1.607 s after the pelican has started it's descent. The pelican has gone down:
s = ut + 1/2at2 = 0 + 1/2(-9.80 m/s/s)(1.607 s)2 = -12.65 m and is therefore 16 - 12.65 = 3.34 m above the water.
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