Whats the difference between mechanical energy and thermal energy?

One way to think of this is the mechanical energy is the energy in the mechanical parts.  The gears, motors, or bodies in motion.  The thermal energy is released as a result of the work done by the mechanical parts.  This energy of heat is released into the surround atmosphere.


The law of Conservation of Energy states that the total energy of a system is conserved.

Therefore if we have an initial system say a car's engine.  The energy in that system will be the same in the beginning as it is in the end.  The say the motor in the car moves.  This kinetic motion causes thermal energy.  The energy is then transferred from mechanical to to thermal.  This is what heats up the engine.  The total energy though remains the same.  There may be less kinetic energy on the gears, but more thermal energy in the surrounding particles.

Well the textbook is wrong.  It's correct if and only if the potential energy has a mechanical source.  Then ME = KE + PE.  But see, the deal is this.  Potential energy can come from, say, chemicals, like those in a flashlight battery.  And that PE is not mechanical PE.

Examples of mechanical PE are gravity PE where a body has potential energy due to its location in a gravity field and spring stored energy when a coiled spring is stretched or compressed and holds PE from that.

At the atomic or molecular level, thermal energy is based on the root mean squared average KE of all the particles.  The more rms KE they have, the hotter they will feel.  I know of no thermal PE however.  Even unused thermal energy in storage and not being used is the result of rms KE.  So as far as I am concerned all thermal energy is rms KE, it's all kinetic.