题目

Wind is the great maker of waves. There are exceptions, such as the tidal (潮汐的) waves sometimes produced by earthquakes under the sea. But the waves most of us know are produced by winds blowing over the sea. Now before constructing an imaginary life history of a typical wave, we need to know certain physical things about it. A wave has height, from trough (low point) to crest (high Point). It has length--the distance from this crest to that of the following wave. The period of the wave means the time it takes for succeeding crests to pass a fixed point. None of these things stay3．小球以v0=6m/s的速度从中间滑上光滑的足够长斜面，已知小球在斜面上运动时的加速度大小为2m/s2，问小球速度大小为3m/s时需多少时间？（提示：小球在斜面上运动时加速度大小、方向不变）