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笔试试题 (共六题,总分120分,150分钟完成) 一、(本题25分) 有一长为l,重为W_0的均匀杆AB,A端顶在竖直的粗糙墙壁上,杆端与墙面间的静摩擦系数为μ;B端用一强度足够而不可伸长的绳悬挂,绳的另一端固定在墙壁C点,木杆呈水平状态,绳与杆的夹角为θ,如图1所示。 1.求杆能保持平衡时μ与θ应满足的条件; 2.杆保持平衡时,杆上有一点P存在,若在A点与P点间任一点悬挂一重物,则当重物的重量W足够大时总可以使平衡破坏;而在P点与B点之间任一点悬挂任意重量的重物,都不能使平衡破坏。求出这一P点与A点的距离。二、(本题20分) 如图2所示:一质量均匀分布的细圆环,其半径为R,质量为m。令此环均匀带正电,总电量为Q,现将此环平放在绝缘的光滑水平桌面上,并处于磁感应强度为B的均匀磁场中,磁场方向竖直向下。当此环绕通过其
Pen questions (a total of six questions, a total score of 120 minutes, 150 minutes to complete) A, (25 points in this question) There is a long rod l, weight W_0 uniform rod, A end on the vertical rough wall, rod end with The coefficient of static friction between the walls is μ; the B-end is suspended with a rope of sufficient strength but not extensible, and the other end of the rope is fixed at the point C of the wall. The wood is horizontal and the angle between the rope and the rod is θ, as shown in the figure. 1 shows. 1. Find the condition that μ and θ should satisfy when the rod can maintain balance; 2. When the rod is in balance, there is a little P on the rod, if a weight is hung at any point between A and P, the weight of the weight When W is large enough, the balance can always be destroyed; and if any weight is suspended at any point between points P and B, the balance cannot be destroyed. Find the distance between this point P and point A. Second, (20 points in this question) As shown in Figure 2, a thin ring of evenly distributed mass has a radius of R and a mass of m. Let this ring be evenly charged with a positive charge of Q. This ring is now placed flat on an insulated, smooth horizontal table and in a uniform magnetic field of magnetic induction B, with the magnetic field directed vertically downwards. When this surrounds it