Question 2.CE.3: Diagram Jeopardy The force diagram shown here describes the ......
Diagram Jeopardy
The force diagram shown here describes the forces that external objects exert on a woman (in this scenario, the force diagram does not change with time). Describe three different types of motion that are consistent with the force diagram.
Sketch and translate Two equal-magnitude oppositely directed forces are being exerted on the woman (\Sigma \vec{F}=0) . Thus, a motion diagram for the woman must have a zero velocity change (\Delta \vec{v}=0) .
Simplify and diagram Three possible motions consistent with this idea are shown at the right.
1. She stands at rest on a horizontal surface.
2. She glides at constant velocity on rollerblades on a smooth horizontal surface.
3. She stands on the floor of an elevator that moves up or down at constant velocity.
Note that in all three of the above, the velocity change arrow is zero. This is consistent with the sum of the forces being zero.
Try it yourself: Suppose that the elevator described above was moving up at decreasing speed instead of at constant speed. How then would the force diagram be different?
Answer: A velocity change \Delta \vec{v} arrow for her motion would now point down opposite the direction of her velocity. Thus, the sum of the forces \Sigma \vec{F} that other objects exert on her must also point down. This means that the magnitude of the upward force \vec{F}_{\text {S on } \mathrm{W}} that the elevator floor (surface) exerts on her must now be less than the magnitude of the downward force \vec{F}_{\text {E on } W} that Earth exerts on her \left(F_{\mathrm{S} \text { on } \mathrm{W}}<F_{\mathrm{E} \text { on } \mathrm{W}}\right) .
