文档介绍：Statics
静力学
物体的受力分析
PART3 Free-Body Diagram
of a body
Bilingual Edition
Free-body diagram
The first step in equilibrium analysis is to identify all the forces that act on the body, this is plished by means of a Free-body diagram
Free-body diagram: The free-body diagram (FBD) of a body is a sketch of the body showing all forces that act on it. The term free implies that all supports have been removed and replaced by the forces (reactions) that they exert on the body.
1. Free-body diagram
PART THREE
Importance of FBD
Free-body diagrams are fundamental to all engineering disciplines that are concerned with the effects that forces have on construction of a FBD is the key step that translates a physical problem into a form that can be analyzed mathematically.
2. Importance of FBD
PART THREE
Theory of unlocking the constraint
If the body is in equilibrium under the acting of some positive forces, its status of equilibrium will not change when parts or all the constraints have been removed and replaced by the corresponding constraint forces.
3. Theory of unlocking the constraint
PART THREE
Steps of drawing FBD
Select the part of study (or all the rigid body system)
Separate the part (all) from the body system
Draw all the positive forces
Erase the constraints, draw the corresponding reaction forces.
5. Steps of drawing FBD
PART THREE
Sample problem 1
The homogeneous 6-m bar AB is supported by rollers at A and B and by a cable at C, The mass of the bar is 50kg. Draw the FBD of bar AB, Determine the number of unknowns on the FBD.
PART THREE
Sample problem 1
Solution:
Step1:chose the bar AB, separate it from the body system
Step2:draw the positive force: the weight of the bar W=mg=50×=490N
Step3:draw the normal reactions at point A and point B
Step4:draw the tension in the cable at C
We note that there are three unknowns on the FBD: the magnitude of the three reactions(F NA F NB F TC ).
PART THREE
Sample problem 2
Draw the FB