帮我生成解说视频---**Extraction Content:** **Question Stem:** 9. 如图, 关于 x 轴对称的光滑导轨固定在水平面内, 导轨形状为抛物线, 顶点位于 O 点。一足够长的金属杆初始位置与 y 轴重合, 金属杆的质量为 m, 单位长度的电阻为 r₀。整个空间存在竖直向上的匀强磁场, 磁感应强度为 B₀。现给金属杆一沿 x 轴正方向的初速度 v₀, 金属杆运动过程中始终与 y 轴平行, 且与电阻不计的导轨接触良好。下列说法正确的是 ( )。 **Translation of Question Stem:** 9. As shown in the figure, smooth tracks symmetric about the x-axis are fixed in a horizontal plane. The shape of the tracks is a parabola, with the vertex at point O. A sufficiently long metal rod is initially at the position coinciding with the y-axis. The mass of the metal rod is m, and its resistance per unit length is r₀. A uniform magnetic field exists throughout the entire space, vertically upward, with magnetic induction intensity B₀. Now, the metal rod is given an initial velocity v₀ along the positive x-axis. The metal rod remains parallel to the y-axis throughout its motion and is in good contact with the resistance-free tracks. Which of the following statements is correct? **Options:** A. 金属杆沿 x 轴正方向运动过程中, 金属杆中电流沿 y 轴负方向。 B. 金属杆可以在沿 x 轴正方向的恒力作用下做匀速直线运动。 C. 金属杆停止运动时, 与导轨围成的面积为 (mv₀r₀)/B₀². D. 若金属杆的初速度减半, 则金属杆停止运动时经过的距离小于原来的一半。 **Translation of Options:** A. During the process of the metal rod moving along the positive x-axis, the current in the metal rod is along the negative y-axis. B. The metal rod can move in uniform linear motion under the action of a constant force along the positive x-axis. C. When the metal rod stops moving, the area enclosed by the tracks is (mv₀r₀)/B₀². D. If the initial velocity of the metal rod is halved, the distance traveled by the metal rod when it stops is less than half of the original distance. **Chart/Diagram Description:** * **Type:** A 2D coordinate system with a physical setup involving tracks, a moving bar, and a magnetic field. * **Coordinate Axes:** X-axis pointing horizontally to the right, Y-axis pointing vertically upwards. The origin is labeled O. * **Tracks:** Two curved lines, symmetric about the X-axis, starting from the origin O and extending to the right. These represent smooth parabolic tracks. * **Moving Bar:** A vertical line segment parallel to the Y-axis, representing the metal rod. It is shown at two different positions along the X-axis. The initial position appears to be at the Y-axis (x=0). * **Velocity:** An arrow labeled v₀ points along the positive X-axis at the initial position of the rod, indicating the initial velocity. * **Magnetic Field:** Represented by multiple black dots scattered uniformly across the area shown, indicating a uniform magnetic field pointing out of the page (vertically upwards in the 3D perspective described in the text). The label B indicates the magnetic field. * **Points:** The origin is labeled O. Point B is labeled near the upper track at the second position of the rod.