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Rotational Motion Test-Taking Tips


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Our AP-C class just had our multiple-choice/FRQ unit test on rotational motion, and with 20:20 hindsight I have a few tips for anyone looking to tackle his or her exam.

I. Know your formulas.

Make sure that you not only know the syntax of each formula, but also the meaning behind it. Formulas are always a representation of a relationship between two or more variables, and sometimes they will be tested in this specific manner. For example, with the formula Force(centripetal) = (mv^2)/r, the formula itself is important, but the fact that an increase in radius decreases the amount of centripetal force is equally as important.

II. Remember what you have, and use it to find what you need.

Many complex FRQs, and some multiple choice, cannot be solved with the use of only one formula. Sometimes one simple conversion is required to bridge the gap between two realms of givens, especially in rotational motion. Remember that all of the kinematics formulas can be converted into their rotational equivalents. An example of "bridging the gap" is found in a problem where two blocks are hung over opposite sides of a pulley with non-negligible mass. First, the net force in both the x- and y-directions must be found. Then the rotational motion of the pulley must be accounted for in a new torque calculation. Finally, the angular acceleration must be converted to translational acceleration to relate the two types of motion.

III. Use units to your advantage.

Before panicking over a confusing multiple choice question, remember that there is more to an answer than the number. If the units differ between answers, attempt to find what the units should be using your memorized formulas.

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Some excellent points... and you're absolutely right, in a majority of the problems we've worked through from old AP exams, our "bridge" equations were the translation to rotational kinematic equivalents!

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