It is difficult to teach and learn the question of the acting of variable force.

In order to evaluate energy parameters of propellant accurately and impersonally, thermodynamic parameters of different propellant formulations were computed by potential energy method, and the correlations between the thermodynamic parameters and expanding power capability of propellant combustion gas in barrel weapon were researched.

According to the work of magnetic force,the relationship between induction electro motive force and the energy change in magnetic field is analysed.

According to the characters of the work of magnetic force,the concept of potential energy can be brought in the non conservative system-magnetic field.

In this paper,the power problem of completely wave rectifier is discussed by many methods respectively,such as harmonic analyses.

A paradox of work done by friction is clarified, and the general format of principle of energy-work and the first law of thermodynamics are presented.