Lesson 3 Forces and Moments 力和力矩 机械专业专业英语教学用书.pptVIP

Lesson 3 Forces and Moments 力和力矩 When a number of bodies are connected together to form a group or system, the forces of action and reaction（作用力和反作用力）between any two of the connecting bodies are called constrain forces（约束力）. These forces constrain the bodies to behave in a specific manner. Forces external to this system of bodies are called applied forces（外力）. Lesson 3 Forces and Moments 力和力矩 Electric, magnetic, and gravitational forces are examples of forces that may be applied without actual physical contact. A great many, if not most, of the forces with which we shall be concerned occur through direct physical or mechanical contact (实际接触或机械接触). Lesson 3 Forces and Moments 力和力矩 Force F is a vector. The characteristics of a force are its magnitude, its direction, and its point of application. The direction of a force includes the concept of a line, along which the force is directed, and a sense (指向). Thus, a force is directed positively or negatively (正负) along a line of action. Lesson 3 Forces and Moments 力和力矩 Two equal and opposite forces acting along two noncoincident (不重合) parallel straight lines in a body cannot be combined to obtain a single resultant force. Any two such forces acting on a body constitute a couple (力偶). The arm of the couple is the perpendicular distance between their lines of action, and the plane of the couple is the plane containing the two lines of action. Lesson 3 Forces and Moments 力和力矩 The moment of a couple is another vector M directed normal to the plane of the couple; the sense (指向) of M is in accordance with the right-hand rule for rotation. The magnitude of the moment is the product of the arm of the couple and the magnitude of one of the forces. Lesson 3 Forces and Moments 力和力矩 A rigid body is in static equilibrium if: (1)The vector sum of all forces acting upon it is zero. (2)The sum of the moments of all the forces acting about any single axis is zero. Mathematically these two statement