Work done by conservative forces changes one form of mechanical energy into another. Is it correct to assume that work done by non-conservative forces changes one form of energy to another, for e.g., from internal energy to a form of mechanical energy or vice versa?
Also, consider the situation where one moves a object up (case 1) and down (case 2) from a table to the floor. Now, what are the roles of the works done by the two forces (conservative (gravity) and non-conservative (us)) in each case?
As far as I understand, conservative forces cannot change the net energy of the system and only the conservative forces can bring change in potential energy of the system, whereas change in kinetic energy can be caused by both conservative and non-conservative forces.
In case 1, where we move the object up, the system (object and earth) gains potential energy and this energy was supplied from us. Now, the work done by us causes the transfer of chemical energy from us to the kinetic energy and the work done by the gravitational force converted this kinetic energy into potential energy such that $\Delta K=0$. Also, the change in chemical energy in us would be greater than the energy transferred to the system since some chemical energy gets converted into heat inside us and this cannot be regained. And in this case, there is no change in internal energy of the system (or is it possible for the work done by us to transfer some of our chemical energy to the internal energy of the system?).
In case 2, where we move the object down, the potential energy decreases and we don't gain that energy. The work done by the gravitational force converts the potential energy into the kinetic energy and the work done by us converts this kinetic energy (and some of our chemical energy) into internal energy of the system (and us) such that $\Delta K=0$. Here, the increase in the internal energy of the system is equal to the decrease in its potential energy (or greater than it, if the work done by us also transfers some of our chemical energy to the internal energy of the system).
Is this right? Nobody explains it in this way. Correct me if I'm wrong.
Best Answer
I think a mathematical approach may help you.
Change in Potential energy of a system is defined as the negative of work done by the internal conservative forces of the system $$dU_{system}=-dW_{int,con}$$
Work-energy theorem states that $$dW_{total}=dK_{system}$$
There may be internal and external forces present in the system,then $$dW_{total}=dW_{int,con}+dW_{int,non-cons}+dW_{external}$$
Simplifying this equation further $$dW_{total}=-dU_{sys}+dW_{int,non-cons}+dW_{external}$$
It immediately follows from the above equation that
$$dW_{int,non-cons}+dW_{external}=dU_{system}+dK_{system}$$
We know that the $RHS$ of the above equation is nothing but $dE_{mechanical}$,then $$dE_{mechanical}=dW_{int,non-cons}+dW_{external}$$
It is now clear that only the the work done by the internal non conservative and external forces can change the mechanical energy of a system.