We next consider the example. A constant horizontal force F pulls a block along an x axis and through a displacement of magnitude d, increasing.
By experiment we find that the block and the portion of the floor along which it slides become warmer as the block slides. As we shall discuss in Chapter 19, the temperature of an object is related to the object’s thermal energy E’h (the energy associated with the random motion of the atoms and molecules in the object). Here, the thermal energy of the block and floor increase because (I) there is friction between them and (2) there is sliding.
Is the work W done by the external force F (the energy transferred by the force). but on which system is the work done (where are the energy transfers made)’? To answer, we check to see which energies change.
This equation, which is represented is the energy statement for the work done on a system by an external force when friction is involved.