# How to justify Mechanical Works doesn't involve heat variations?

My professor while teaching Mechanical energy said all the mechanical works are performed at at a constant temperature and said this is the main difference between the thermodynamic work and mechanical work that thermodynamic work typically involves work and heat transfer. Taking an example of motor, so far I know, we say it cannot perform at 100% efficiency some work is converted to heat. How can we justify the statement?

• I think your professor is trying to explain the difference between mechanical energy, which is kinetic and potential energy of an object as a whole, and internal energy, which is kinetic and potential energy of the molecules of an object. A temperature increase when performing mechanical work would mean some mechanical energy has been lost to internal energy. Apr 18, 2022 at 15:14

The statement: the work done by the net force on an object is equal to the change of the kinetic energy ($$E_k$$) is always valid.
When the force is conservative (as gravity for example), there is a well defined conserved quantity $$E = E_p + E_k$$, where $$E_p$$ is called potential energy. And there is no temperature change involved.