A student picks leftward as the positive direction at the start of a motion problem, then wants to switch to rightward halfway through. What is the correct rule?
AThe positive direction may be flipped at any convenient step
BOnce a positive direction is assigned it cannot be changed
CThe positive direction must always be taken as rightward
DThe positive direction may change once the sign of v changes
Answer & Solution
Correct answer: B. Once a positive direction is assigned it cannot be changed
1. Any consistent choice of positive direction is allowed at the outset.
2. Leftward may be chosen as positive, for example when runners race to the left.
3. Once a positive direction is assigned and the problem is under way, it cannot be changed.
4. Switching midway would give some signs one meaning and other signs the opposite meaning.
_Source: OpenStax College Physics (CC BY 4.0), Ch 2 "Kinematics", section 2.2 Vectors, Scalars, and Coordinate Systems_
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