Some families argue politics or religion. AP Physics readers have been arguing free body diagrams.Many good teachers have differing opinions about the purpose and proper construction of the free body diagram. The grading of free bodies on the AP exam has been the subject of extremely lively debate for many years. Everyone, fortunately, has common goals:
* Award credit to students for correct physics
* Do NOT award credit to students for vague or incorrect physics
* Have enough flexibility in the rubric to accomodate alternative but reasonable interpretations of the correct construction of the free body diagram.
Perhaps the loudest of the shouting involves the issue of vector components. Many of us -- including me -- take it as a matter of faith that a free body should never include components of a force. Others reasonably point out that since the whole point of the diagram is to determine the magnitudes of the forces, and since one must break down angled vectors into components at some point, such components represent a necessary and important part of problem solving technique. I see the legitimacy to both arguments.
In order to quiet the arguments and send a clear message about free body components, the AP development has decided to change the language used to ask students to draw free body diagrams. The test questions now will SPECIFICALLY forbid components on free body diagrams, and will even remind students to use a separate space to draw components where necessary. You can see the white paper issued by the development committee here within the College Board's website.
Moral of the story: Argument settled by fiat. DO NOT put components on free body diagrams, or you will lose points. Break angled vectors into components on a diagram separate from the initial free body.
so no
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ReplyDeleteExcept for the "Normal force" and "friction force" exerted by a ramp on a box. Those are merely two perpendicular "components" of a singular force of ramp-on-box.
ReplyDeleteFor the case of, say, a box sliding down a ramp at constant speed, I presume that AP readers would award full credit for a clearly labeled FBD that had 1 downward force of Earth-on-box and 1 upward force of ramp-on-box, both of the same magnitude.
It seemed a little inconsistent to me that AP says "no credit for FBD components, ever!" but then gives full credit to students who break out the ramp-on-box force into parallel "friction" and perpendicular "normal" components.
Unknown, I see your point here - we've had long discussions about this issue, too. Thing is, every textbook in the universe (hard copy or online) treats the friction force and the normal force as separate entities. You're of course correct that these are, using technical language, the components of the force of the ramp on the box, one parallel to the ramp, one perpendicular.
ReplyDeleteNevertheless, because textbooks, online physics sources, and the College Board's CED itself all treat friction and normal forces as separate, individual forces, students are likewise expected to separate these. Weird if you're a professional physicist, I know; but absolutely normal (wordplay intended!) if you're a student of introductory physics.