- #1

fog37

- 1,568

- 108

Just refining my understanding of the work-KE theorem and seeking some validation:

- The work-kinetic energy theorem states that the mechanical work done by a force (be it conservative or nonconservative) is always equal to the change in kinetic energy of the body.
- The net force in a dynamic problem is a force given by the vector sum including both conservative and non conservative forces. A sum of just conservative forces produces a net conservative force but a sum of conservative and nonconservative forces must certainly be a nonconservative net force, correct?
- ONLY in the case of conservative forces, the change in kinetic energy ##\Delta{KE}## can be equal to the negative change in potential energy ##\Delta{PE}##: ##\Delta{KE}= \Delta{PE}##, correct?
- A nonconservative force, when it performs nonzero work, can ONLY change the kinetic energy of the body but not the potential energy so it does not make sense attributing a change in ##PE## to a nonconservative force. Is that all there is not understand?
- Example: let's consider a rocket climbing vertically up at constant speed. There are two forces acting on the rocket: gravity pointing down and thrust ##F_{thrust}## pointing up. The thrust force pushes a rocket upward and performs work (force times distance). Gravity also performs work. But neither works causes a ##\Delta{KE}##.
- The net force is zero, the net work is zero, so ##\Delta{KE}=0##. The thrust force, when separately analyzed, does produce work but not a change in ##KE## nor ##PE##. However, gravity produces no change in ##KE## but a change in ##PE##.
- Potential energy, which is energy of configuration (relative position between system parts), is therefore a form of energy that can only be affected by conservative forces. Potential energy ##PE## only depends on the mutual distance between the system's components. So while kinetic energy is frame of reference dependent, ##PE## is not since the mutual distance between system parts is not affected by choosing different reference frames. Could we even talk about the potential energy of a system (not the change in ##PE##) if there were no conservative forces acting on the system but only nonconservative forces?