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At night, Babe would make noises and hit the ground with his feet.
VOA: special.2009.01.24
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We want to know, as you can tell-- our concern is where does the car hit the ground?
我们想知道,你们也能算出来,车将在哪一点落到地面
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you can hit the ground running, as we say.
也就是说，你可以立刻进入状态。
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Mr.Ban says he will visit some of the hardest hit areas, including the conflict zone, to assess conditions on the ground .
VOA: standard.2009.05.19
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If you want to know something like when do you hit the ground, I think it's fairly clear what you have to do.
如果你想知道车子何时落地,我想你该做什么就很明显了
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and you've got to hit a ball which is on the ground with a sort of croquet mallets.
你要用一个类似木槌的东西去打击地上的球。
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When does the car hit the ground?
车什么时候落地
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Again, let t* be the time when you hit the ground.
设 t* 为落地的时间
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When does it hit the ground and at what speed?"
该物体何时落地,落地速度是多少
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When does it hit the ground" is "When is y=0"?
何时落地"也就是"何时位移为零"
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So when does it hit the ground?
那么何时落地
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For example, if t* is the time you hit the ground, then t* satisfies the equation h - 1/2 g ^2 = 0.
例如,如果在 t* 时刻落地,那 t* 就满足等式 h - 1/2 g ? ^2 = 0
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Conversely, if you get an answer and it doesn't seem to make sense, then you've got to go back and ask, am I violating some of the assumptions, and here you will find the assumption that the particle had that acceleration a is true as long it's freely falling under gravity but not when you hit the ground.
反过来说,如果你得到一个结果,发现似乎是错的,那么你就应该回过头来问问自己,我是不是违反某些前提了,这个模型中,你就发现,只要质点在重力作用下自由下落,质点具有加速度a的前提是正确的,但是落地后就不成立了
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