form 3 IS 一問 20點 急!!!

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.

Mechanical energy=Kinetic energy + Potential energy
Through a motion of a force acting on a body, making the body to move,
Work is done for energy is changed from mechanical energy into other forms of energy(mostly heat).

For the rubbing motion, the body is moving with the presence of frictional force.
Thus work is done while kinetic energy should be decrease,as if the body is moved by sth. (human hand). The kinetic energy of the body would be kept while the
chemical energy inside us would be consumed.

In fact, work done sometimes equal the energy loss of the body.
In F.4 Level, Mechanics has a formula : Work = Force * distance