Doing work on a system can have very different consequences. FIGURE 19.2a shows an object being lifted at steady speed by a rope. The rope’s tension is an external force doing work $W_{\text{ext}}$ on the system. In this case, the energy transferred into the system goes entirely to increasing the system’s macroscopic potential energy $U_{\text{grav}}$, part of the mechanical energy. The energy-transfer process $W_{\text{ext}}\to E_{\text{mech}}$ is shown graphically in the energy bar chart of Figure 19.2a.

FIGURE 19.2 The work done by tension can have very different consequences.

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Contrast this with FIGURE 19.2b, where the same rope with the same tension now drags the object at steady speed across a rough surface. The tension does the same amount of work, but the mechanical energy does not change. Instead, friction increases the thermal energy of the $\text{object}+\text{surface}\text{system.}$ The energy-transfer process $W_{\text{ext}}\to E_{\text{th}}$ is shown in the energy bar chart of Figure 19.2b.

The point of this example is that the energy transferred to a system can go entirely to the system’s mechanical energy, entirely to its thermal energy, or (imagine dragging the object up an incline) some combination of the two. The energy isn’t lost, but where it ends up depends on the circumstances.