这个物理题是怎么写的？---**题目** 质量为$8g$的子弹，以$300m/s$的速度射入 厚度为$5cm$的固定木板（如图），射穿后 的速度是$100m/s$。子弹射穿木板的过程 中受到的平均阻力是多大？ **Chart/Diagram Description:** * **Type:** Illustration/Diagram showing a physical process. * **Main Elements:** * A vertical rectangular object representing a wooden board. It is depicted as fixed. * A projectile (bullet) shown approaching the left side of the board from the left. * The bullet is shown passing through the board, with wood chips/fragments depicted flying off from the point of entry. * The bullet is shown exiting the right side of the board and moving away to the right. * A dashed representation of the bullet is shown partially inside the board, indicating its path through the material. * Arrows are not explicitly drawn for velocity, but the positioning implies motion from left to right, entering and exiting the board. * **Relative Position and Direction:** The bullet is shown moving horizontally towards and through the vertical board. The board is depicted as stationary and fixed. The bullet enters from the left, passes through, and exits to the right.