英语翻译A tennis ball is thrown vertically upward with an initia

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英语翻译
A tennis ball is thrown vertically upward with an initial velocity of +8.0m/s.
(a).What will the ball's speed be when it returns to its starting point?
(b).How long will the ball take to reach its starting point?
小疑问：如果初速度是＋8.0m/s,往上是匀减速运动,到达最高点时速度应是0m/s.球往下掉时,而且加速度在这整个往上和往下的过程不都应该是－9.8m/s^2

将一网球以+8.0m/s的速度垂直上抛
问（a）网球落回抛出点时的速度是多少?
(b)从抛出到落回到抛出点整个过程费时多少?
解
（a）不计空气阻力,从抛出点到再落回来,整个过程是对称的,所以落回的速度大小为抛出的速度的大小,只不过是方向相反,即v2=-8.0m/s
（b）因为：根据时间也对称：所以有,
t=2*(0-v1)/g=16/9.8=1.63 s 取 g=-9.8m/s^2
你的想法都是对的.

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