求问

细细青颜 · 发表于 2010-12-29 09:42:05

DS：一个球从斜坡上滚下来，斜坡的长度d与球从坡顶滚到坡地的时间t满足d=t平方。问能否求出球在第一秒内的平均速度和最后一秒内的平均速度之差。
（1）已知坡长25feet
（2）已知总共用了5s

细细青颜 · 发表于 2010-12-29 09:43:32

我觉得1和2都行啊。
坡长=25可以求出t啊。。
求牛牛解答

sdcar2010 · 发表于 2010-12-29 09:54:54

Yeah, you are right.

D is the answer.

sdcar2010 · 发表于 2010-12-29 09:59:00

This is simple. Both conditions are the same since 25= 5^2.

At the end of the first second, d = 1 ft. So the v1 average = 1/1 = 1 ft/s
At the end of the 4th second, d = 16 ft. At the end of the 5th second, d = 25 ft. So during the last second, the ball covers 25-16 = 9 ft. So the v5 average = 9/1 = 9 ft/s.

The different between the two is 8 ft/s.

Choose D.

求问

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