Class 8 - Internal Forces in a Truss Class 08 9/25/2007...

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9/25/2007 ENGR 111 Track A 1 Internal Forces in a Truss Class 08
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9/25/2007 ENGR 111 Track A 2 Learning Objectives In this module you will learn how to Draw the free body diagram of the  joints  in a truss. Develop the equilibrium equations for a  joint. Apply the equilibrium equations to find the unknown  forces at a  joint . Identify whether a  member  is in tension or compression.
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9/25/2007 ENGR 111 Track A 3 Two force members Demo
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9/25/2007 ENGR 111 Track A 4 Objective  This is a demo of a series of actual physical  experiments to show how rigid bodies that are  subject to only two forces will behave at equilibrium. At the end of this demo, you should be able to: 1. Draw the FBD of any two force member. 2. Write down the reason why such bodies have  such a simple FBD at equilibrium.
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9/25/2007 ENGR 111 Track A 5 Straight bar with two forces Consider the black Lego bar  with the two strings attached  lying on a HORIZONTAL  PLANE. Note that the strings  are parallel.    1. What will happen if I apply  tension to the two strings (i.e. I  pull on the strings)? 2. How many forces are acting  on the bar in the plane?  Click on Picture to watch clip
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9/25/2007 ENGR 111 Track A 6 Do you understand the initially  the string tension exerts a  TORQUE on the bar? Did you observe that the bar  rotated until the string was  aligned? Did you understand that there is  NO torque on the bar when the  strings are aligned? Please watch the video on the  previous page again to clarify  these points.
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9/25/2007 ENGR 111 Track A 7 Bar with two perpendicular forces Consider the bar with two  forces that are not aligned. What do you think will happen  to the bar if the strings are  pulled? Watch video clip by clicking
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9/25/2007 ENGR 111 Track A 8 Did you understand that the  string exerts forces on the bar  that are not in equilibrium? Did you see that the bar  translates and rotates until the  strings are aligned? Please watch video clip on the  previous page again with these  things in mind. 
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9/25/2007 ENGR 111 Track A 9 Test your understanding In each case, predict what will happen.  Then play the video by clicking on the image
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9/25/2007 ENGR 111 Track A 10 Key concepts When body is subject to only  TWO (2) Forces in equilibrium,  the forces will be aligned so that  (1) their magnitudes are equal  (2) their directions are opposite  (3) their LINE OF ACTION will be  the same.
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Class 8 - Internal Forces in a Truss Class 08 9/25/2007...

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