A woman waiting to cross the street holds a briefcase in her hand

as shown in ▶ Figure 4.12a. Identify all of the third law force pairs

involving the briefcase in this situation.

▲ FIGURE 4.12 Force pairs of Newton’s third law (a) When a person holds a briefcase, there are two force pairs: a contact pair (F1 and −F1) and an action-at-a-distance (gravity) pair (F2 and −F2).The net force acting on the briefcase is zero: the upward contact force (F1 on the briefcase) balances the downward weight force. Note, however, that the upward contact force and downward weight force are not a third law pair. (b) Any third law force pairs? See the Follow-Up Exercise. REASONING AND ANSWER. The briefcase is being held motionless,

so its acceleration is zero, and ΣFy =

Transcribed Image Text:

Contact forco on briefcase Action-at-distance fres (b)