# Free body diagram and force vectors

• samjohnny
In summary, the person is trying to figure out how to draw a free body diagram for the system and is having trouble understanding how to do it.
samjohnny

## Homework Statement

Attached, where AC=BC=2R and CD=CE=3R. I need to work out the force the cylinder exerts on the cross section (in order to work out the moment at C). So first I'm trying to work out the free body diagrams for this system. Is it as I've attached? How do I draw the tension vector? And then I'm having a hard time defining and decomposing the force vectors in terms of theta, mg etc. Can anyone lend a hand?

#### Attachments

• forces.JPG
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• 20140310_204405.jpg
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• 20140310_204418.jpg
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hi samjohnny!

we need to know a little more …

are CD and CE hinged at C?​
samjohnny said:
So first I'm trying to work out the free body diagrams for this system. Is it as I've attached? How do I draw the tension vector?

it would be easier to start with the free body diagram of the cylinder

your puzzlement as to how to draw the tension is caused by choosing the wrong body

a free body diagram shows all the external forces on a body, so it won't show up on a body for which it's an internal force!

you need to do it for one rod

(the symmetry makes that a lot easier than it would otherwise be)

tiny-tim said:
hi samjohnny!

we need to know a little more …

are CD and CE hinged at C?​
it would be easier to start with the free body diagram of the cylinder

your puzzlement as to how to draw the tension is caused by choosing the wrong body

a free body diagram shows all the external forces on a body, so it won't show up on a body for which it's an internal force!

you need to do it for one rod

(the symmetry makes that a lot easier than it would otherwise be)

Yeah they're connected at C. Cool, I see what you mean. I'll take another crack at it.

## What is a free body diagram?

A free body diagram is a graphical representation used to analyze the forces acting on a single body or object in a given system. It shows the magnitude and direction of all the forces acting on the body, and is an important tool in solving problems involving forces and motion.

## How do you draw a free body diagram?

To draw a free body diagram, first identify the object or body of interest and draw it as a dot or box. Then, identify and draw all the forces acting on the body, including the weight, normal force, friction, and any other relevant forces. Finally, label the forces with their corresponding magnitudes and directions.

## What is the purpose of a free body diagram?

The purpose of a free body diagram is to visualize and analyze the forces acting on a body in a given system. It helps in understanding the motion of the body and determining the net force acting on it. Free body diagrams are often used in physics and engineering to solve problems involving forces and motion.

## What are force vectors?

Force vectors are mathematical representations of forces, which include both magnitude and direction. They are typically represented by arrows, with the length of the arrow representing the magnitude of the force and the direction of the arrow indicating the direction of the force. Force vectors can be added and subtracted to determine the net force acting on a body.

## How do you calculate the net force on a body using free body diagrams and force vectors?

To calculate the net force on a body, first draw a free body diagram and identify all the forces acting on the body. Then, assign a positive or negative sign to each force based on its direction. Finally, add all the forces using vector addition to determine the net force on the body. If the net force is equal to zero, the body is in a state of equilibrium, meaning that all the forces are balanced.

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