[工学]中国石油大学工程力学课件tm101sta.ppt

* Kylinsoft Statics-1-* * Kylinsoft Statics-1-* 视频1、视频2、视频3 * Kylinsoft Statics-1-* * Kylinsoft Statics-1-* Special Cases: Two-Force Bodies and Three-Force Bodies ① Two-force bodies(二力构件)If a body is held in equilibrium by two forces, the forces must be equal in magnitude and oppositely directed along the same line of action. 如棘爪,不计自重时为二力构件 * Kylinsoft Statics-1-* ② Three-force bodies(Three-force Principle) Three nonparallel, coplanar forces that hold a body in equilibrium must be concurrent. * Kylinsoft Statics-1-* Sample Problem 1-1 用力F拉动压路的碾子。已知碾子重P，并受到固定石块A的阻挡，如图(a)所示。试画出碾子的受力图。 Solution: 1.研究对象:碾子。解除约束，画出碾子的轮廓图，如图1-26(b)。 2.　画上主动力:拉力和重力． 3．画上约束力：碾子在A、B两点受到石块和地面的约束，约束力分别为FNA和FNB。不计摩擦，约束力都沿接触点的公法线而指向碾子的中心。碾子的受力如图（b）所示。 在碾子即将越过石块的瞬时，其受力图有何变化呢？ 注意到：此时碾子将在B 处脱离约束，约束力FNB消失，即FNB=0 。 * Kylinsoft Statics-1-* Sample Problem 1-2:  Draw FBD of rigid frame AB. * Kylinsoft Statics-1-* 1-3 * Kylinsoft Statics-1-* Sample Problem 1-4Draw FBD of the beam AB. * Kylinsoft Statics-1-* Sample Problem 1-5 The weight of motor is P2，and the weight of beam AB is P1 ,neglect the weight of bar BC. Draw FBD. Solution： 1. 确定研究对象. 如研究BC受力,取杆BC为研究对象。 2. BC单独作图. 3. 画上主动力: BC上无主动力. 4. 画约束力: 由于杆重不计，且只在两端受力而平衡，故为二力构件，有 FBC =- FCB 其受力如图(c)所示。 再取梁AB和电机为研究对象。主动力有梁的自重P1和电动机的重力P2 。约束力有FBC’，且FBC’ = - FBC 。铰链A的约束力方向未知，以FAx和FAy表示。梁和电机系统的受力如图(c)所示。 * Kylinsoft Statics-1-* Sample Problem 1-6:  Draw FBD of AC, BC and ABC. * Kylinsoft Statics-1-* Sample Problem 1-7: 简易起重机如图, 不计各杆和滑轮重量, 画出各研究对象的受力图： (1)重物连同滑轮; (2) DE杆；(3) BC杆; (4) AC杆; (5) AC杆连同滑轮和重物; (6) 整体. * Kylinsoft Statics-1-* Sample Problem 1-8 A structure with applied forces is shown in Fig.. Draw the FBD for the structure and for each components. The weights of AB and the weight of BC are all W. * Kylinsoft Statics-1-* 这里Bx，By及Bx’，By’称为系统内力（internal reactions)。 思考: 如销钉位于某个物体上时如何作FBD? 如果P=0,Q=0 又如何? * Kylinsoft Statics-1-* 1-9 * Kylinsoft Statics-1-* 1-10 * Ky