解答这道题---**Textual Information:** **Question Stem:** 如图，一半径为R、粗糙程度处处相同的半圆形轨道竖直固定放置，直径POQ水平。一质量为m的质点自P点上方高度R处由静止开始下滑，恰好从P点进入轨道。质点滑到轨道最低点N时，对轨道的压力为4 mg。g为重力加速度的大小。用W表示质点从P点运动到N点的过程中克服摩擦力所做的功。则 **Options:** A. W = (1/2)mgR, 质点恰好可以到达Q点 B. W > (1/2)mgR, 质点不能到达Q点 C. W = (1/2)mgR, 质点到达Q点后，继续上升一段距离 D. W < (1/2)mgR, 质点到达Q点后，继续上升一段距离 **Other Relevant Text:** None explicitly mentioned outside the question stem and options. **Mathematical Formulas/Chemical Equations:** W = (1/2)mgR W > (1/2)mgR W < (1/2)mgR 4 mg g **Table Content:** None. **Chart/Diagram Description:** * **Type:** Schematic diagram showing a vertical semi-circular track and the path of a point mass. * **Main Elements:** * **Points:** Point mass 'm' shown above P. Point P, O, Q are on a horizontal line (diameter). Point N is the lowest point of the semi-circular track. O is the center of the semi-circle. * **Lines:** A vertical arrow indicates a distance R from the initial position of mass 'm' to point P. A horizontal dashed line connects P, O, and Q, representing the diameter. A semi-circular curve connects P and Q through N, representing the track. A line segment from O to N is shown, labeled R, representing the radius. * **Shapes:** A semi-circle. * **Coordinate Axes:** Not explicitly shown, but the setup implies a vertical and horizontal direction. * **Data Points/Series:** Not applicable to this type of diagram. * **Angles:** The line POQ is horizontal, and the track is vertical, implying O-N is vertical and perpendicular to POQ. * **Labels and Annotations:** 'm' labels the point mass. 'R' labels the initial height above P and the radius of the track. 'P', 'O', 'Q', 'N' label specific points. * **Relative Position and Direction:** The mass starts at a height R above P. P, O, Q are collinear and horizontal. N is vertically below O. The semi-circular track connects P and Q, passing through N. * **Legend:** None.