# 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.

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

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

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

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

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 ??