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• #### Index 355-366

 BOX 5–7 Use of Multiple-Choice Questions to Test for Theoretical vs. Knowledge-in-Pieces Perspective Problem: In the following situation, two identical steel marbles M1 and M2 are to be launched horizontally off their respective tracks. They each leave the ends of their respective tracks at the same time, but M2 will leave its track travelling twice as fast as M1. The track for M1 can be set at any height in relation to M2. If we want the two marbles to collide, how will we need to arrange the horizontal launch tracks? The track for M1 should be much higher than the track for M2. The tracks for M1 and M2 should be at the same elevation. The track for M1 should be much lower than the track for M2. Now, suppose we have set the track for M1 at an appropriate height so that the marbles will collide in the space between and below the two tracks. The picture below is just before the two marbles collide. Diagram (arrows with appropriate relative lengths) and label the forces on each marble. [A for horizontal component in the forward direction] [B for no horizontal component in the forward direction] When they collide, which marble will exert the greater force on the other? M2 will exert the greater force on M1. M1 will exert the greater force on M2. M2 and M1 will exert equal forces on each other. Briefly justify your answer. After the marbles collide, they fall the rest of the way to the floor. Which would reach the floor the soonest? They will reach the floor at the same time. M1 will reach the floor before M2. M2 will reach the floor before M1. Briefly justify your answer.

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