When I raise a pen, the external work I do gets stored in the form of potential energy hence the mechanical energy is increased. When I leave the pen the pen starts falling and as the potential energy decreases the kinetic energy increases. No external force so no change in mechanical energy.
When the pen strikes the ground, it's kinetic energy is brought to zero and its potential energy is also decreased. Hence, its total mechanical energy is reduced. But the ground only exerted a normal force on the pen, it didn't do any work. No other external force acted upon the pen. There was a change in mechanical energy even though no external force did any work on it. the (7) point given in the image is contradicted. Where am I going wrong?
I even wanted to ask what we mean by non-conservative internal forces?