问题标题：
SAT2物理题Awomanstandingonatrainplatformlistenstoatrainasitapproachesherataconstantspeed.Thetrainengineerblowsthewhistleofthetrainwhichheknowstoproduceafrequencyof450HZ.Asthetrainapproachesthewomanonthep
问题描述：
SAT2物理题
Awomanstandingonatrainplatformlistenstoatrainasitapproachesherataconstantspeed.Thetrainengineerblowsthewhistleofthetrainwhichheknowstoproduceafrequencyof450HZ.Asthetrainapproachesthewomanontheplatform,shewillhearthefrequencyofthewhistleas()A.greterthan450HZbutconstantB.greaterthan450HZandsteadilyincreasing为什么选A呢?为什么是constantapproach应该是increasingfrequency
马群回答：
因为火车接近的话听到的频率会比450大.火车是匀速运动所以虽然是变大但是频率还是CONSTANT,如果是火车是加速运动的话F就是INCREASING了
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