1. The problem statement, all variables and given/known data We got a ball moving at speed 2/√3 m/s, and it hits another ball that is attached to the string. Ball 2 has 3 times the mass of ball 1. After the crash, ball one stucks in ball 2 and the spring gets .02m shorter at its minimum length. There are frictions with n=1/12 and g=10m/s^2. Find the constant k of the spring. 2. Relevant equations ΔP=0=>mu=4mu', u'=u/4, you cant use hooke's law as it isnt in the textbook. Fy=0<=>N=mg M=3m, M'=4m 3. The attempt at a solution I used conservation of energy principle to get 1/2Mu'^2=1/2kx^2-nM'g=>Mu'^2+2nM'g=kx^2=>(Mu'^2+2nM'g)/x^2=k and thn just plug in numbers, but the problem is I ended up with a k as a function of m.