Item 10 requires students to reason about the combined effect, or superposition, of two gravitational forces. With
a difficulty of 0.79, this question is quite complex because it also requires proficiency in the Force Law Domain
via proportional reasoning with both mass and distance. Student interviews indicate that the most effective
distractor, chosen more often than the correct choice, represents the idea that proportional changes to mass and
distance are weighted equally in determining force. Interviews also suggest that the remaining distractors are
chosen by students who reason with intuition rather than proportional reasoning. These na€ıve intuitions include
the idea that objects can experience gravitational force from only one other object at a time, or simply that
objects are pulled toward either the closer or the more massive object. Figure 2 shows that, despite its difficulty,
this question is actually a very good discriminator of student understanding.
Item 10 requires students to reason about the combined effect, or superposition, of two gravitational forces. With
a difficulty of 0.79, this question is quite complex because it also requires proficiency in the Force Law Domain
via proportional reasoning with both mass and distance. Student interviews indicate that the most effective
distractor, chosen more often than the correct choice, represents the idea that proportional changes to mass and
distance are weighted equally in determining force. Interviews also suggest that the remaining distractors are
chosen by students who reason with intuition rather than proportional reasoning. These na€ıve intuitions include
the idea that objects can experience gravitational force from only one other object at a time, or simply that
objects are pulled toward either the closer or the more massive object. Figure 2 shows that, despite its difficulty,
this question is actually a very good discriminator of student understanding.
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