So in my internet readings on Lagrangian mechanics I started researching applications with non-potential and/or non-conservative forces and came across this page: http://farside.ph.utexas.edu/teaching/336k/Newton/node90.html This page is fascinating but I'm having a bit of difficulty understanding a piece of the first example. Can some one explain to me the constraint function they came up with? Its labeled as equation 727. I feel like I'm missing something obvious but I just can't figure it out. Thank you!