Contradictory FBD's for simple pulley problem

In summary, the conversation discusses a basic pulley problem where a mass is suspended from a massless pulley attached to the ceiling. When creating a free-body-diagram by 'cutting' at different points, there is confusion about the tension in the ropes. It is eventually realized that the tension in the top rope is equal to twice the weight of the mass.
  • #1
mordechai9
205
0
I have a basic pulley problem which has been troubling me.

Consider the system as shown in the attached picture. We have a massless pulley attached to the ceiling and a mass suspended from one side of the pulley.

If we take the free-body-diagram by 'cutting' where the red box is, we find the tension in the bottom rope is equal to the tension in the top rope. But if we take the free-body-diagram by 'cutting' at the green box, then we find the tension in the top rope is twice the tension in the bottom rope, since the tension in the rope is everywhere the same.

What is going wrong here?
 

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  • #2
Nevermind. I just forgot for the red-box FBD you still have to include the weight of the mass, so actually it just says the tension in the top rope is equal to twice mg again. Doh. Too long since I did basic physics.
 

1. What is a "contradictory FBD" in a simple pulley problem?

A "contradictory FBD" refers to a free body diagram that does not accurately represent the forces acting on an object in a simple pulley problem. This means that the net force calculated from the diagram does not match the actual net force acting on the object.

2. Why do contradictory FBD's occur in simple pulley problems?

Contradictory FBD's can occur due to a misunderstanding of the forces involved in a simple pulley system, incorrect labeling of the forces, or not accounting for all the forces acting on the object.

3. How can I avoid contradictory FBD's in simple pulley problems?

To avoid contradictory FBD's, it is important to have a clear understanding of the forces involved in the pulley system. Make sure to label all the forces correctly and account for all the forces acting on the object, including tension, weight, and friction.

4. How can contradictory FBD's affect the accuracy of my calculations?

If contradictory FBD's are used in calculations, the results may be inaccurate and misleading. This can lead to incorrect predictions and conclusions, which can affect the overall validity of the experiment or study.

5. What steps can I take to correct a contradictory FBD in a simple pulley problem?

If you have identified a contradictory FBD, go back and check your understanding of the forces involved and make sure all forces are accurately labeled. If necessary, redraw the FBD or consult with a colleague or mentor to verify the accuracy of your diagram.

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