NASA Free-Fall Research: Solving Problems with Kinematic Equations

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I have two problems. I have filled in most of the information that is asked but I can't seem to figure out a part for each problem. Please Help.

Problem 1:

Homework Statement



At a NASA research center free-fall research is performed by dropping experiment packages from the top of an evacuated shaft 146 m high. Free-fall imitates the so called microgravity environment of a satellite in orbit.
a) what is the max time interval for free fall if an experiment package were to fall the entire 146 m?

5.45 s [correct]

b) Actual NASA specs allow for a 5.24 s drop interval. How far do the packages in 5.24 s?

135m [correct]

c) What is their speed at 5.24 s?

51.5 m/s [correct]

d) What constant acceleration would be required to stop an experiment package in the distance remaining in the that shaft after its 5.24 fall?

121 m/s^2 [incorrect]

Homework Equations



Kinematic equations


The Attempt at a Solution



Presumably, since the shaft is 146 m tall and the package has already fallen 135 m there are 11 m left for the package to stop.

deltaX = 11 m : Vi = 51.5 m/s : Vf = 0 m/s

this information should be enough to calculate Ax for the second part of the problem but Webassign says my answer is wrong. Any ideas guys?

Problem 2

Homework Statement



An inquisitive physics student and mountain climber climbs a 48.0 m cliff that overhangs a calm pool of water. He throws two stones verticall downwar, 1.00 s apart and observes that they cause a single splash. The first stone has an initial speed of 1.92 m/s downward. Assume the downward direction is positive.

(a) How long after release of the first stone do the two stones hit the water?

2.93 s [correct]

(b) What initial velocity must the second stone have if they are to hit simultaneously?

16.1 m/s [incorrect]

(c) What is the speed of each stone at the instant the two hit the water?

first stone

30.7 m/s [correct]

second stone

34.62 m/s [correct]


Homework Equations



Kinematic equations


The Attempt at a Solution



This seems like a pretty straightforward problem but I can't seem to get question B right even though I used the value for question B to get the correct answer for question D.
 
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b)The package is accelerating down wards with g. Your answer is true if the package is moving with uniform velocity. So net acceleration should be ...?
 
131 m/s^2 in the upward direction to compensate for g, which is incorrect according to Webassign