1. The problem statement, all variables and given/known data . A 2.0 kg object falls from rest a distance of 5.0 meters onto a 6.0 kg object that is supported by a vertical massless spring with spring constant k = 72 N/m. The two objects stick together after the collision, which results in the mass/spring system oscillating. What is the maximum magnitude of the displacement of the 6.0 kg object from its original location before it is struck by the falling object? (A) 0.27 m (B) 1.1 m ← CORRECT ANSWER (C) 2.5 m ← what im getting (D) 2.8 m (E) 3.1 m 2. Relevant equations Conservation of mechanical energy(spring, gravitational, and kinetic) conservation of momentum for the inelastic collision 3. The attempt at a solution I'm just copying the solution here because it's what I did, but I will point out where i did something different. Conserve energy to find the speed of the 2.0 kg object at the instant before the collision: m1gy = (2.0 kg)(10 m/s)(5.0 m) = 100 J and this equals the kinetic energy, after plug and chug we get v = 10.0 m/s The collision is inelastic, so vf = v1*m1/(m1+m2) = 2.5 m/s Now conserve energy again Kf = 1/2mtv2 = 1/2(8.0 kg)(2.5 m/s)2 = 25 J This compresses the spring and changes the gravitational potential energy, so we must solve for x in the equation 1/2kx2 + mtgx = Kf or 36x2 + 20x − 25 = 0 I used 8kg as the total mass instead of 2kg, why did they use 2kg? which has solutions x = −1.15 m and x = 0.6 m. solutions with mt=8kg: x = -2.5, 0.27. I'm a bit reluctant to say this, but i think aapt might have made a mistake on the answer key. On top of my confidence that my solution is right, they already used mt=8kg in the previous step before apparently changing its value in the next step. Now, given I'm a mere high schooler, i would still like someone to double check my work so that i can ask aapt to make the small fix.