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I'm having promblem with b. I checked the solution, but I did'nt understand the logic.
Kf+Ugf+Usf = Ki+Ugi+Usi
Usf= 0, Ki=0, Ugf= 0
I understand the logic's why they are equal to 0.
I don't understand why final potential of spring (Usf) is equal to zero?
Now, I got it. I read the problem in detail.