# Drawing FBD: Cable Passes Over a Smooth Peg

• Engineering
• xxxyzzz
In summary, the problem asks for the FBD for a cable hanging from a point C which is not attached to it, and asks for the equivalent point load to the distributed load. The problem also asks for the reaction force at C which does not exist.
xxxyzzz
Homework Statement
Determine the resultant internal loadings on the cross-section at point E. The cable passes over a smooth peg at C.
Relevant Equations
N/A

Attempting to draw the FBD for this problem but was wondering what to do about the cable, especially when I determine the internal loadings at E. At C, do I only draw one force arrow (CB), or draw both on the left and right side?

xxxyzzz said:
At C, do I only draw one force arrow (CB), or draw both on the left and right side?
You draw ALL of the forces extant at that point. How many are there?

Chestermiller and berkeman
xxxyzzz said:
The cable passes over a smooth peg at C.
...
At C, do I only draw one force arrow (CB), or draw both on the left and right side?
Welcome!
The cable, just like it happens to any rope or any chain, can only pull, it can't push anything.
In order to pull an object, it needs to be solidly attached to it.
Please, note that the problem states that the cable is not attached to the point C, it can only freely slide over it.

Hello, thank you for the welcome and responses. I am unsure if the notifications will be received if I do not click on the reply button, but I drew my idea of the FBD below and what I think all the forces are. I did not think that the cable is only pulling. Is this a correct assumption of the forces?

And to find the internal loadings at E, do I proceed with this?

Thanks all.

I believe that the equivalent point load to the distributed one should not be 12 KN.
What Ve stands for?

hutchphd
Lnewqban said:
I believe that the equivalent point load to the distributed one should not be 12 KN.
What Ve stands for?
My apologies, I screenshotted the problem from the textbook where w=3 kN/m, while my homework problem had w=4 kN/m. It should be 9 kN in my FBD. Ve is meant to be the internal shear force at E.

No apologies needed.
Force at C is incomplete.
If the structure is in equilibrium, the summation of forces and moment acting on each of its elements must be zero.

Draw the ENTIRE FBD all at one time with ALL of the forces. Always do that when solving this kind of problem

Lnewqban
https://www.physicsforums.com/attachments/313781Are these all the forces? I did not think that C had any reaction forces because it is smooth.

First, your link is broken and second PUT IT RIGHT HERE. This is like pulling teeth.

phinds said:
First, your link is broken and second PUT IT RIGHT HERE. This is like pulling teeth.
I was editing my post since I was unsure about my diagram, which was probably why the link was broken (might have accidentally deleted it). I believe I forgot some basic concepts from Statics since it has been a while, so I am having trouble understanding what forces I am missing. I do not mean to make you feel like you are "pulling teeth." Would like to ask you to be patient with me. Here is the reupload.

xxxyzzz said:
I was editing my post since I was unsure about my diagram, which was probably why the link was broken (might have accidentally deleted it). I believe I forgot some basic concepts from Statics since it has been a while, so I am having trouble understanding what forces I am missing. I do not mean to make you feel like you are "pulling teeth." Would like to ask you to be patient with me. Here is the reupload.
View attachment 313784
Check your math on the distributed load. The reaction force at C, doesn’t exist.

Last edited:
xxxyzzz
xxxyzzz said:
View attachment 313784
This diagram is much better.
Time to compare forces and moments to determine the value of the cable tension Fbc.
With that value on hand, a vectorial addition of the tensions (same value, different directions) acting on C should let you know what types and what magnitudes of loads the cross-section E of the vertical element is withstanding.

@xxxyzzz, Did you have time to solve this problem?

## What is a free body diagram (FBD)?

A free body diagram (FBD) is a visual representation of the forces acting on an object. It is a simplified drawing that shows the object as a dot and all the forces acting on it as arrows. It helps to analyze and understand the motion of an object.

## How do you draw a FBD for a cable passing over a smooth peg?

To draw a FBD for a cable passing over a smooth peg, you first need to identify the object and all the forces acting on it. The object in this case is the cable and the forces acting on it are the tension force from the cable and the normal force from the peg. Draw a dot to represent the cable and draw arrows to represent the direction and magnitude of these two forces.

## Why is it important to draw a FBD for this scenario?

Drawing a FBD for a cable passing over a smooth peg helps to understand the forces that are acting on the cable and how they affect its motion. It also helps to identify any unknown forces that may be present and to determine the net force acting on the cable.

## What are some common mistakes when drawing a FBD for this scenario?

One common mistake is not including all the forces acting on the object. For example, forgetting to include the normal force from the peg. Another mistake is not correctly representing the direction and magnitude of the forces, which can lead to incorrect analysis of the motion of the object.

## Can a FBD be used for other scenarios besides a cable passing over a smooth peg?

Yes, a FBD can be used for any scenario where there are forces acting on an object. It is a useful tool for analyzing the motion of an object in various situations, such as a car on an incline or a ball being thrown in the air.

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