Our question to solve is: A 5.2 g Ball bearing is fired vertically downward from 18m with an initial speed of 14 m/s. It buries itself a depth of 21 cm in the sand. What average resistance force does the sand exert on the ball bearing? We know the answer is -6.81N but we are having a hard time figuring out which equations to use to solve the problem. Please help! Thanks!