1. The problem statement, all variables and given/known data A rock is rolled in the sand. It starts at 5.0 m/s, moves in a straight line for a distance of 3.0m and then stops. What is the magnitude of the average acceleration? 2. Relevant equations aavg = change in V/ change in time magnitude of acceleration = change of speed/time interval. 3. The attempt at a solution i think i am thinking too hard about this problem. seems simple but cannot get the right answer found time = 1.67 s and tried to plug in for average acceleration. 5.0/1.67 = 2.99 m/s^2 the correct answer is 4.2 m/s^2. ?? can't figure out what i am doing wrong.