1. The problem statement, all variables and given/known data This is a quiz question that was given in my AP physics C class. "A rock is launched upward with a speed V0. With what velocity must a second rock be launched from the same initial position at time T later so that it hits the first rock when it has a velocity of -V0/2." 2. Relevant equations The only formulas we needed for this question were the basic velocity, displacement, and acceleration formulas (as functions of time) and their variations. (1D kinematics). 3. The attempt at a solution 1) I drew a diagram on an axis with a vertical line (y axis) and the origin being zero (the initial position of the balls). 2) I listed all of the knowns and unknowns given from the question. We know that: Rock1 (first rock) has an initial velocity of V0. Time= T Acceleration= -g= -9.81 m/s2 When the second rock hits the first rock, the velocity of the first rock will be -V0/2. We don't know the velocity that is necessary for rock2 (second rock), which allows the given condition ( When the second rock hits the first rock, the velocity of the first rock will be -V0/2.) to happen. Basically, we don't know the initial velocity of rock2. I then assumed the conditions for the collision: When the rocks collide, they will be the same distance from the ground, so d1=d2 and I also assumed the time of travel for both rocks at the collision, which I put as T=T, which is where I'm sure that I made a mistake. Since this is from a collected quiz, I don't remember exactly what I did. However, I remember doing something similar to this: (-V0rock1 /2)T- (1/2)gT2=( V0rock2)T- (1/2)gT2 I forgot the process, but my solution was something like V0/2T, which is incorrect because the units come out to be m/s2. I feel that I did something completely off track, and I really need help as to how I should approach these problems and how to think more effectively. I felt as though this question should have been easier for me, which kind of discouraged me after that quiz was collected. This isn't the first time we get a question like this, and help for the question and questions like these would be much appreciated. Thank you in advance.