第章 空间力系.pptVIP

1 * * 第五章 空间力系 Chapter 5 space forces system 5–4 空间平行力系的中心 ·物体的重心 the centriod of space parallel force system ,the center of gravity of the body 5–1 力在空间坐标轴上的投影 the projections of forces in space coordinate axis 5–2 力对轴的矩 the moment of a force about an axis 5–3 空间力系的合成与平衡 the reduction and equilibrium of space force system 引课 guiding 迎 面风 力 侧 面风 力 b Analysis all the forces acted at point A and the car. We find that the acting lines of all forces are distributed in different planes. 1、一次投影法（直接投影法） The method of direct projection 5-1 力在空间坐标轴上的投影 the projections of forces in space coordinate axis 2、二次投影法（间接投影法） The method of indirect projection 当力与各轴正向夹角不易确定时，可先将 F 投影到xy面上，然后再投影到x、y轴上 When the angle between the axis and the force is determined uneasy , first determine the projection to the plane of xy ,then determine the projections to the axis . 5-2 力对轴的矩 the moment of a force about an axis 一、力对轴的矩的概念与计算 the concepts and the calculation of the moment of a force about an axis 定义：definition 力F与轴共面时，力对轴之矩为零。 The moment of a force about an axis equals zero when the force is coplanar to the axis. Sign convention Clockwise is positive Counterclockwise is negative 力对轴的矩是力使刚体绕该轴转动效应的度量，是代数量，其大小等于在垂直于转轴的平面内的分量的大小和它与转轴间垂直距离的乘积，其正负号按右手规则确定。 The moment of a force about an axis tends to the body rotate about the axis, it is algebraic element. Its magnitude equals the components of the force in a plane times the distance from the components to the axis. the sign convention is based on the right-hand rule. [例1]example 1 已知：P=2000N, C点在Oxy平面内。 求：力P对三个坐标轴的矩。 To determine the moments of force P about coordinate axis. 解：solution 1、几何法：与平面汇交力系的合成方法相同，也可用力多 边形方法求合力。 即：合力等于各分力的矢量和 一、空间汇交力系的合成： reduction of space current force system 5-3 空间力系的合成与平衡 the reduction and equilibrium of space force system Geometry methods By using the polygon rule to