Your approcah doesn't work because it doesn't comply with the facts given. For the deceleration phase you know s = 0.008m, but you don't know t (you tried to use 1 second but that is not the time of deceleration) and at the instant the ball hits the sand its velocity is not 0 but rather v=gt, where t = 1 second.
Mu suggestion is (1) calculate the velocity of the ball when it hits the sand, then (2) find the rate of deceleration using the equation v_2^2 - v_1^2 = 2ad, where v_2 is the final velocity (0), v_1 is the initail velocity (calculated in step 1), and d = 0.008m.
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Last edited by ChipB; Jan 10th 2013 at 03:17 PM.
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