Elementary Principles: D'Alembert's Principle and Lagrange's Equations

Timestamps for equations:

Eqs. 1.40-1.42: 0:12
Eq. 1.43: 1:50
Eqs. 1.44-1.45: 2:55
Eqs. 1.46-1.47: 4.38
Eqs. 1.48-1.49: 6:09
Eq. 1.50: 8:10
Eq. 1.51: 9:31
Eq. 1.52: 11:30
Eqs. 1.53-1.55: 12:45
Eqs. 1.56-1.57: 14.20