# Kinematics bird dive question

Petrikovski
Pelicans tuck their wings and free-fall straight down when diving for fish. Suppose a pelican starts its dive from a height of 16 m and cannot change its path once committed. If it takes a fish .20 s to perform evasice action, at what minimum height must it spot the pelican to escape? Assume the fish is at the surface of the water.

## Answers and Replies

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Fermat
Homework Helper
The pelican fall from a height H, say. When it is at a height h, it is spotted by the fish. 0.2s later, the pelican strikes the water, and the fish.

Very similar to this problem here, but with a change in unknowns.

Petrikovski
i solved that one but i have another 2 problems :|

A stone is thrown vertically upward with a speed of 22 m/s. (a) How fast is it moving when it reaches a heihgt of 15 m? (b) How long is required to reach this height? (c) Why are there two answers to b?

for (a), using X = 15, Xsub0 = 0, Vsub0 = 22, and a = -9.8 i got 13.78 m/s. For b i got .84 seconds. Those are both right but, for (b), theres a second answer becausre the ball comes back down past 15 M. but i cant figure out how to get it. The answer is 3.65 seconds but idk how to get it. any help? this ones really puzzling me

A person jumps from a fourth-story window 15 m above a firefighters safety net. The survivor stretches the net 1 m before coming to rest. (a) What was the average deceleration experienced by the survivor when slowed by the net? (b) Would tightening the net or loosening it increase the deceleration?

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Fermat
Homework Helper
Petrikovski said:
...

for (b), theres a second answer becausre the ball comes back down past 15 M. but i cant figure out how to get it.

...
Should should have a quadratic in t when solving for time for the particle to be at a height of 15m

$$x_f = x_0 +v_0*t - \frac{1}{2}gt^2$$

That'll give you both answers.

Petrikovski
Fermat said:
Should should have a quadratic in t when solving for time for the particle to be at a height of 15m

$$x_f = x_0 +v_0*t - \frac{1}{2}gt^2$$

That'll give you both answers.
ok i got it but still need help with other one

A person jumps from a fourth-story window 15 m above a firefighters safety net. The survivor stretches the net 1 m before coming to rest. (a) What was the average deceleration experienced by the survivor when slowed by the net? (b) Would tightening the net or loosening it increase the deceleration?

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Fermat
Homework Helper
The person falls for 15m under gravity.
What is his velocity at this point.
He then travels 1m before coming to a stop - he has final velocity = 0
That's all you need to apply one of the eqns of motion - vf, vi, accln and distance travelled.
For (b), would you rather jump into a taut net or a flaccid net ??