The school principal had been observing Ms. M. during this class and asked her to explain why she had not followed her original lesson plan. Ms. M. explained that the girl had made a similar statement to the class twice before. Ms. M. realized that the girl was not being disruptive but was making a legitimate point that the other members of the class were not grasping. So Ms. M. decided that continuing with the discussion of motions and forces would not be fruitful until the class had developed a better concept of frame of reference. Her questions were designed to help the students realize that motion is described in terms of some point of reference. The insect in the middle of lily pad would describe its motion and the motion of the spider in terms of its reference frame, the lily pad. In contrast, the bird watching from the edge of the stream would describe the motion of the lily pad and its passengers in terms of its reference frame, namely the ground on which it was standing. Someone on the ground observing the bird would say that the bird was not in motion, but an observer on the moon would have a different answer.



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