# Conservation of mechanical energy when object strikes the ground

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?

• Non-conservative forces are those forces which cannot be expressed as the gradient of a scalar potential and thus have non-zero curl. – user36790 Jun 2 '16 at 10:55
• @MAFIA36790 understand the concept of conservative and non-conservative forces but what is a non-conservative internal force? – oshhh Jun 2 '16 at 11:00
• Internal forces are those between parts of a system. External forces are those between some part of the system and something that is not part of the system. A "non-conservative internal force" is an internal force that isn't conservative. – dmckee --- ex-moderator kitten Jun 2 '16 at 22:35

For a conservative force the work done in going from position $A$ to position $B$ is independent of the path taken.When the pen hits the ground the ground is deformed.
For a non conservative force the work done does depend on the path taken and the frictional force is an example of such a force. If you slide a block from position $A$ to position $B$ on a flat table the path you take will determine how much work you need to do to move the block.