# Help with free body diagram

by pyroknife
Tags: body, diagram, free
 P: 418 For the attached picture I need to draw a free body diagram for the structure supporting the pulley. At the fixed end (left), there would be a vertical and horizontal reaction force as well as a moment. I am not sure what the forces and moments would be for the part connected to the pulley. What would it look like for this situation? Attached Thumbnails
 P: 616 You're shown some of the forces. What others are missing?
P: 418
 Quote by tms You're shown some of the forces. What others are missing?
I did not show any forces in the picture.

There should be a force where the pulley is attached, but I don't know what direction that should be in.
Edit-I think there should be a reaction force where the pulley is connected in 'both' the x and y direction.

 P: 418 Help with free body diagram There should also be a reaction moment where it's connected.
P: 616
 Quote by pyroknife I did not show any forces in the picture.
What about the string that runs over the pulley?
P: 418
 Quote by tms What about the string that runs over the pulley?
The 2 forces shown are given. They act on the pulley. The problem is to draw a free body diagram for the structure.
 P: 616 Does the pulley exert any forces on the structure? You've said the wall exerts a reaction force on the beam? What is it reacting to?
 P: 418 The wall holds the cantilever beam. It exerts a horizontal and vertical (maybe) force on the structure. It also exerts a moment because it's cantilevered. The pulley is being held by the structure, so yes, it exerts a force which should be both in the horizontal and vertical direction. My question is if this is right and if there is a moment exerted as well.
 Mentor P: 5,379 There is no moment at the pulley, because the pulley bearings prevent this. But there is a vertical force and a horizontal force exerted by the pulley axis (spindle) on the structure.
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 Quote by pyroknife The wall holds the cantilever beam. It exerts a horizontal and vertical (maybe) force on the structure. It also exerts a moment because it's cantilevered. The pulley is being held by the structure, so yes, it exerts a force which should be both in the horizontal and vertical direction. My question is if this is right and if there is a moment exerted as well.
Yes, there is a moment.

Are the bracket holding the pulley, and the pulley itself massive enough so the their masses have to be considered?
 P: 418 Thank you. It seems Chestermiller and SammyS may have conflicting opinions on whether there should be a moment exerted where the pulley is attached. "Are the bracket holding the pulley, and the pulley itself massive enough so the their masses have to be considered?" I believe the structure is what's the pulley. I have as much info as you guys, so I'm not entirely sure. Typically for shear and moment diagram (which this is) problems, we ignore mass. This is what I assumed since they don't give you mass. I think I agree with Chestermiller that there isn't a moment exerted at where the pullley is.
 P: 418 Attached is what I think the free body diagram should look like. Where the 4 arrows indicate the arbitrary reaction forces and the counter clock wise arrow represents the moment. Attached Thumbnails
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 Quote by pyroknife Thank you. It seems Chestermiller and SammyS may have conflicting opinions on whether there should be a moment exerted where the pulley is attached. "Are the bracket holding the pulley, and the pulley itself massive enough so the their masses have to be considered?" I believe the structure is what's the pulley. I have as much info as you guys, so I'm not entirely sure. Typically for shear and moment diagram (which this is) problems, we ignore mass. This is what I assumed since they don't give you mass. I think I agree with Chestermiller that there isn't a moment exerted at where the pullley is.
There's a moment exerted at the junction of the structure and the wall. (I thought that's what the question was asking.)
P: 418
 Quote by SammyS There's a moment exerted at the junction of the structure and the wall. (I thought that's what the question was asking.)
Oh no, I mean where the pulley and structure were connected. It was a dumb question.
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 Quote by pyroknife Oh no, I mean where the pulley and structure were connected. It was a dumb question.
I agree that the two forces in the figure produce a zero moment about the pulley axis.
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 Quote by pyroknife Attached is what I think the free body diagram should look like. Where the 4 arrows indicate the arbitrary reaction forces and the counter clock wise arrow represents the moment.
You have correctly shown the force and moment reactions at the fixed end, but at the pulley end, although you have correctly identified the 2 forces acting there, you have incorrectly shown the direction of those forces, and you have not indicated their magnitudes. Their direction and magnitudes must be properly shown on the FBD of the frame. The direction and magnitudes of the forces on the frame at the pulley can be determined by first drawing a FBD of the pulley, and then applying Newton's 3rd law.

When you talk about 'arbitrary' reaction forces and moment at the wall, I am not sure what you mean. You can arbitrarily choose their direction, I suppose, and their true directions will be determined when the equlibrium equations are applied. But their is no arbitation on their magnitudes (unknown until the equilibrium equations are applied), and no arbitration whatsoever in the magnitude and directions of the forces at the pulley end.
 P: 263 If the reactions are drawn in the wrong direction, then their value will come out negative. The magnitude should still be the same. And don't say that a beam exerts a moment because its cantilever, it exerts a moment because the fixed support prevents it from wanting to rotate. What if the beam was supported by a roller or pin? Neither one of those supports have a moment reaction
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P: 5,379
 Quote by caldweab If the reactions are drawn in the wrong direction, then their value will come out negative. The magnitude should still be the same. And don't say that a beam exerts a moment because its cantilever, it exerts a moment because the fixed support prevents it from wanting to rotate. What if the beam was supported by a roller or pin? Neither one of those supports have a moment reaction
A beam with a roller or pin is not cantilevered. The definition of a cantilever beam is one that is "built in" (to a wall).

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