✔ 最佳答案
Here, the "mechanical energy" refers to "kinetic energy", which is the energy possessed by an object in motion.
The example has clearly explained the meaning of the sentence. The rubbing action of your hand possesses "mechanical energy". Through the action of friction , the rubbing action just turns such mechanical energy into heat, and which raises the internal energy (vibrational energy of molecules in the body on which you rubbed) and temperature of the body.
In the above process, it is said in physics that your hand has done "work" by a process (the rubbing) to transfer mechanical energy (the motion of your hand) into other form of energy (the internal energy of the body). This phenomenon is true in general that work needs to be done when mechanical energy is made to turn into other forms of energy.