1,two trains A and B are traveling on lines which cross at r

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1,two trains A and B are traveling on lines which cross at right angles.A travels at 40 km/h,and B at 50 km/h.find the velocity of B relative to A.
2,A superman warks east at 42 km/h.He suddenly walks south at 26 km/h.What is his change in velocity graw a vector diagram .
第一题，为什么这就是他们的相对速度？这道题如果是find the velocity of A relative to B 结果一样吗？
第二题望解释

A相对B的速度为A的速度－B的速度.矢量减.A相对B的速度的意思是,以B为参考系,对于B来说A的速度是多少（以B的眼光来看A）.原先说的A和B的速度都是相对某个惯性系的,则若以B为参考系,原惯性系做与B的速度等大反向的运动,即原惯性系相对于B做这样的运动,而A又相对于原惯性系有速度A,则A相对于B的速度为A相对原惯性系的速度＋原惯性系相对于B的速度（即A的速度＋负B的速度,即A的速度－B的速度）.（这里说的都是矢量加减）.有很多种理解方式,关键在于惯性系的转换,要以原惯性系为媒介.其实这问题你自己想像理解,我说的只是一种理解方式.这样A相对B的速度和B相对A的速度正好等大反向.第二题速度变化量为末速度－初速度.你可以这样理解,速度的合成即为叠加,我认为宇宙中任何一个速度你都可以把它当成永远存在,只不过物体受到力的作用有加速度,产生一个新的速度,而物体原来那个速度并没有消失,而是物体即走原来那个物体的速度又走产生的新的那个速度,这就是速度的合成,即叠加,叠加后的速度即为物体现在的速度.物体初速度＋速度变化量＝物体现在的速度.这个速度变化量就是产生的那新的一个速度.

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