要想把皮球升上高空,方法有两个:一是直接用力往上抛,二是使劲往下掷,利用反弹的力量使皮球升高;人要想跳过一条比较宽的小河,如果就站在河边起跳,往往不可能跳过,但如果先后退十几步,再用力向前冲,利用加速和惯性,然后奋然一跃,那就必定能跳过了。 To raise the ball to the sky, there are two ways: First, direct upward force throwing, the second is hard to throw down, the use of rebound force to make the ball rise; people want to skip a relatively wide stream, if Standing on the river take off, often can not skip, but if you retreat a dozen steps, and then force forward, the use of acceleration and inertia, and then suddenly leap, it must be able to skip.