Work-KE theorem and net force....

[jbriggs444]

OK, the pucks on the air table...mechanical energy is conserved while it is not for the blocks with friction...

So, it IS important for the internal forces to be conservative for ME to be conserved!
Therefore, for ME to be conserved, two conditions must be satisfied:

a) The internal force pairs must be conservative
b) The net external force is either zero or conservative

A zero force does count (trivially) as conservative.

 

[A.T.]

Therefore, for ME to be conserved, two conditions must be satisfied:

a) The internal force pairs must be conservative
b) The net external force is either zero or conservative

Isn't that simply all forces then?

 

[fog37]

I guess so :)

 

[fog37]

In general, when discussing conservation of mechanical energy for a system, the system is considered isolated which implies that no mass or energy can enter or exit the system. Isolated means that the system does not interact with the environment in any fashion. Lack of external interaction means lack of external force. The net external force $F_{net}$ must be automatically zero if the system is isolated since the system cannot receive or lose energy. I see how a net force can add or subtract energy to the system. How could an external force add or subtract mass to the system?

For an isolated system, energy can only transfer between different parts within the system itself...