Imagine pushing against a brick wall as shown in Figure 9.1a. (Assume the wall is undeformable-a fair assumption at macroscopic scales.)

(a) Considering the wall as the system, is the force you exert on it internal or external?

(b) Does this force accelerate the wall? Change its shape? Raise its temperature?

(c) Does the energy of the wall change as a result of the force you exert on it?

(d) Does the force you exert on the wall do work on the wall?

Transcribed Image Text:

Figure 9.1 An external force does work on a system only if it causes the point at which it is applied (represented by a black dot) to move. (a) Ar=0 Point of application of force (b) Force displacement Art Point of application does not move, Point of application moves, so force does work on cart. so force does no work on wall. (c) mattress- F pm Point of application moves, so force does work on mattress.