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在质点所做匀变速直线运动中,中间位置的瞬时速度 v_+=((v_0~2+v_t~2)/2)~(1/2),中间时刻的瞬时速度 v_+=(v_0+v_t)/2.由数学知识有((v_0~2+v_t~2)/2)~(1/2)≥(v_0+v_t)/2,对匀变速直线运动只能取大于号;只有在匀速直线运动中等号才成立.但在下面情形下,也有 v_+=(v_0+v_t)/2成立的.推导如图1,水平光滑的平行金属导轨左端接有电阻 R,匀强磁场 B 竖直向下分布在导轨所在空间,质量为 m、长为 L 的金属棒 PQ 垂直于导轨放置.今使棒
In the constant-speed linear motion of the particle, the instantaneous velocity in the middle position is v_+=((v_0~2+v_t~2)/2)~(1/2), and the instantaneous velocity in the middle is v_+=(v_0+v_t). )/2. From the knowledge of mathematics ((v_0~2+v_t~2)/2)~(1/2)≥(v_0+v_t)/2, only a greater than sign can be taken for a straight motion with uniform speed; only at a constant speed The movement equal number is established. But in the following situation, there is also v_+=(v_0+v_t)/2 established. The derivation is shown in Figure 1. The horizontal smooth metal rail parallel to the left end has a resistor R, and the uniform magnetic field B is vertical. The metal rod PQ with mass m and length L is placed perpendicular to the guide rail in the space where the guide rail is located.