Rock thrown up from cliff and dropped rock timing problem

[Pseudo Statistic]

Hi.
I'm having problems with some Physics problems; among them:

9. At the top of a cliff a 100 m high, a rock is thrown upward with velocity 15 m/s. How much later should a second rock be dropped from rest so both stones arrive simultaneously at the bottom of the cliff?

a. 5.05 s

b. 3.76 s

c. 2.67 s

d. 1.78 s

2. A 50-gram superball traveling at 25 m/s is bounced off a brick wall and rebounds at 22 m/s. A high-speed camera records this event. If the ball is in contact with the wall for 3.5 ms, what is the average acceleration of the ball during this time interval?

a. 13,428 m/s2

b. 6,715 m/s2

c. 857 m/s2

d. 20 m/s2

For 9), I thought I'd use v = u + at and find 2t after solving:
0 = 15 - 9.8t for t, and that would give me the answer... but I'm wrong.
Anyone know how I can go about this one?
And for 2), I really don't know what to do...

Thanks for any help.

 

[StatusX]

For the first one, your answer was wrong because the first rock will have a downward speed of 15 m/s when it passes you again, and the other rock is dropped from rest. You need to drop the second rock before this point, and the only way to figure out when is to find the total time it will take each rock to hit the ground.

For the second, acceleration is the change in velocity over time. This problem is easier then it seems.