1. The problem statement, all variables and given/known data Mass 1 at 10m/s collides into mass 2 at rest, which has a spring attached to it. The second mass has a spring at 200N/m and natural length L0 = 0.1m. At the instant they collide, the spring is compressed to its max amount and the masses move with the same speed V. Determine the delta X between the objects at this instant. Both momentum and mech energy are conserved. M1=0.4kg M2=0.8kg V1=10 m/s V2=0 (at rest) K= 200N/m L0=0.1m dX=?? 2. Relevant equations Vf=(m1v1+m2v2)/(m1+m2) SPE=0.5Kx^2 KE=0.5mV^2 dX=L-L0 3. The attempt at a solution Vf=(.4)(10)/(1.2)=3.333 So there are three instances here of the collision, when the m1 goes to m2 is the first. Then M1 and M2 being the same object at that one instant. And after. I don't need to measure after so I can just do. KE1+PE1=KE2+PE2 which gives me KE1=.5*m1*v1^2 No contact made at PE1 so theres none here. KE2 = (.5)*(m1+m2)*(Vf)^2 Then PE2 and at this point theres contact and since I know it's compressing the Delta X should be negative (which is still strange) when it all works out. PE2 = (.5)*(200N/m)*(dX)^2 All this will give me KE1=KE2+PE2 to 20m/s = [6.666(Kg)(m^2)/(s^2)] + (.5)*(200N/m)*dX^2 20m/s - [6.666(Kg)(m^2)/(s^2)] = (.5)*(200N/m)*dX^2 .2 m/s - 0.666 (kgm^2)/(s^2) = dX^2 Yes the units don't match up. I can't clear it but if I ignore all that, a -(.466) sqrooted is -.683 as the dX.