1. The problem statement, all variables and given/known data So, I have a 5.0 kg rat that goes down a ski slope, slides along a rough patch, and then smashes into a vertical board with a spring attached. I need to find out the efficiency of the spring with this information: m=5.0 kg v (after sliding across rough patch)=108 m/s ; it also says that because of internal friction, the velocity is 50% than above, so it's 54 m/s (after hitting the board and spring) [tex]\Delta[/tex]x=2.42 m (the magnitude of the spring's compression) 2. Relevant equations e=eout/ein(100%) Possibly Ek=.5mv2 and Eelastic=.5k([tex]\Delta[/tex]x)2 3. The attempt at a solution Well, I think that Ein is equal to Ek, as this is the energy required for the spring to compress, but I'm not completely sure of Eout. Is it possibly Eelastic (I know this value from a previous question)? If I use these two though, I get an answer around 400%, which is obviously wrong.