Hi, all. Here is an expression about kinetic energy from analytical dynamics of Haim Baruh which confuses me. "Consider a particle and the case which the kinetic energy is only quadratic in terms of velocity of the particle. We take the differential form of work-energy principle and remove from it all velocity and time-dependent terms(including the kinetic energy as well as all time-dependent parts of nonconservative force)" Can you expand above explanation please. Here I don't understand why the writer says "case which the kinetic energy is only quadratic in terms of velocity ..." because kinetic energy is always in terms of square of namely quadratic of the linear or angular velocities or sum of both and what are the time dependent terms in the work-energy principle.