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由两个物理量确定的二次方程,通常运用判别式能方便而简捷地确定某个物理量的取值范围,并将取值范围的端点值视为该物理量的极值,但一些参考书将这种方法无条件地推广,以致在方法和结论上都出了问题,这类错误具有一定的普遍性,故有指出的必要。 [典型题目] 质量m=10克的子弹以v_0 =40O米/秒的速度水平射穿一固定木块,射穿后的速度v’=100米/秒。今将此木块(质量为M)改放在光滑水平面上,要使上述子弹仍以水平速度v_0射穿这木块,求木块获
The quadratic equation determined by two physical quantities usually uses the discriminant to easily and simply determine the range of a certain physical quantity, and regards the end value of the range as the extreme value of the physical quantity, but some reference books will This method has been unconditionally promoted, resulting in problems in both methods and conclusions. Such errors have a certain degree of universality and are therefore pointed out. [Typical problem] A bullet of mass m = 10 grams penetrates a fixed block at a speed level of v_0 = 40m/s, and the velocity v’ = 100m/s after the penetration. Now change the block (quality M) to a smooth level so that the above bullet still shoots through the block at a horizontal speed v_0.