打水漂的原理
How does it work
All
right, I guess you are eagle to know why the stone can skip on the surface of
water. As a matter of fact, the operation principle of skipping stones is not
that complicated for us to understand. In order to figure out how does it work,
we should look back to two physical formulas which we have learnt in our junior
high . （在黑板上写1. P=ρgh; 2.
if v1>v2, then p1<p2）If we
cast a stone into the lake, when it drop into the water, because of the
resistance of water, the situation in the lake will be like this. (画示意图，标水流序号) The
water is divided into two parts, as we can see, part one is above the stone and
part two is beneath the stone. When it comes to the pressure of this point, we
can work out the result: P0=ρgh0.
What should be paid attention to is when the stone goes on its way in the
water, it forces the water to flow, thus part one has a velocity v1
and part two has a velocity v2. However, because of the existence of
inertia, the weight of part two is much more than part one, so the velocity of
part two is almost zero. （写v2趋近于0）Now we
learn that the velocity of part one is much higher than part two, according to
the second formula, the pressure of part one is much smaller than part two, so
the part two beneath the stone force it to jump out from the water and then
fall again. So the stone can skip on the surface of lake. In addition, if we
spin out the stone, like this （画示意图）,the when it goes on its way in the water,
it will force the part one to gain a much higher velocity v1 and a
much smaller pressure p1 so that when the stone was forced to jump
out from the water, it will obtain much more energy which enable it to have
much more chances to drop into the water and then jump out from it.
流速越大液压越小。
所以在打水漂的时候应该用比较扁的物体（受力面积大）尽量平行于水面以尽量快的速度扔出去（获得比较的流速），打水漂的物体掠过水面的时候可以带动它上下的水在非常短的时间里快速流动，从而压强减小，而下面的水是静止的产生的压强大，可以把物体压出水面，然后出现第二次或者以此类推出现更多次的物体入水情况。
此外扔出去的物体是旋转的也能取得更好的效果，原因还是使与它接触到的水的流速变快，从而压力减小。
