Free Body Diagram Of a Situation

  • Thread starter Arka2334
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Homework Statement


Hello people,
First of all, this is my first day in this forum and this post is my very first post. So, I am extremely sorry if I violate any rules here.
I have to solve a problem involving frictions. I will solve the problem myself; I just need help with drawing the Free Body Diagram of it. Here is the problem:
http://www.solutioninn.com/engineer...-w1-is-placed-across-the-channel-and-boy-of-w


Homework Equations


F = usN (us is the coefficient of static friction)


The Attempt at a Solution


I have drawn the Free Body Diagram; I would like to make sure if it's correct or not.
Here's what I think is correct. Am I missing anything here?
[PLAIN]https://fbcdn-sphotos-a.akamaihd.net/hphotos-ak-snc7/310450_10150377422927432_808377431_8363863_2056490143_n.jpg [Broken]
 

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Answers and Replies

  • #2
I like Serena
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Welcome to PF, Arka2334! :smile:

In this FBD of the board combined with the boy, you need another force of friction at A.

Btw, the friction at B should be in the opposite direction, since the board would tend to slide down, and the friction on the board opposes that movement.
 
  • #3
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Welcome to PF, Arka2334! :smile:

In this FBD of the board combined with the boy, you need another force of friction at A.

Btw, the friction at B should be in the opposite direction, since the board would tend to slide down, and the friction on the board opposes that movement.
Ahh, thank you very much. Right, the Friction in point B DOES have to be in the opposite side; that was a silly mistake. I was just unsure if I need any force of friction in point A or not, since at that part, the board just seems to be resting on the surface. That was the main reason I asked this question actually.
Anyway, thank you very much for your help. ^_^
 
  • #4
I like Serena
Homework Helper
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Ahh, thank you very much. Right, the Friction in point B DOES have to be in the opposite side; that was a silly mistake. I was just unsure if I need any force of friction in point A or not, since at that part, the board just seems to be resting on the surface. That was the main reason I asked this question actually.
Anyway, thank you very much for your help. ^_^
You're welcome! :smile:

Btw, in an FBD you're supposed to have all the forces, even if some of them may turn out to be zero.
If you're not sure a force is zero, you'd better include it - better safe than sorry.
Only if there are no horizontal forces in play at all can you afford to leave the possible horizontal forces out.
 

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