Which forces am I missing in the FBDs?

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Homework Help Overview

The discussion revolves around a physics problem involving two blocks connected by a massless rope over pulleys on a frictionless surface. The original poster is tasked with drawing free body diagrams (FBDs) for each block and its respective pulley while identifying all forces acting on the systems.

Discussion Character

  • Exploratory, Assumption checking, Problem interpretation

Approaches and Questions Raised

  • The original poster describes the forces they believe are present, including normal forces, weight, and tension forces. They express uncertainty about additional forces that may be missing. Other participants ask for clarification on the original poster's attempts and suggest looking at the ends of the rope to identify forces.

Discussion Status

Participants are actively engaging in clarifying the forces involved in the problem. Some have provided equations based on their interpretations of the FBDs, while others are questioning the completeness of the original poster's analysis. There is a recognition that additional forces may need to be considered, particularly those exerted by the rope on the blocks.

Contextual Notes

The original poster indicates a lack of clarity regarding the forces acting on the systems, particularly concerning the role of the table and the connections of the rope. There is an emphasis on ensuring all forces are accounted for in the FBDs.

JessicaHelena
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Homework Statement


Two blocks with pulleys attached lie on a frictionless surface and are connected by a massless rope strung over the pulleys as shown at right. A force of magnitude F is applied to the second block. Draw a FBD for a system consisting of Block 1 and its pulley, and also a system consisting of Block 2 and its pulley. (See the screenshot attached.)

Homework Equations


F=ma?
I don't think there are any specific equations needed...

The Attempt at a Solution


[/B]
Obviously, there are normal forces by the table on both systems as well as weight, and from each pulley we have two tension forces emanating. The system with block 2 also has an additional force of magnitude F towards the right. However, apparently there are still more forces I am missing, but I can't seem to find them...
 

Attachments

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Unfortunately, your description of what you actually did is too vague for me to understand what you did and what you got. Please provide the equations you obtain from your FBDs with appropriate descriptions of what each term represents.
 
Look at the ends of the rope.
 
What is your attempt at drawing the FBDs?
 
@Chandra Prayaga — please see the screenshot attached.

@Orodruin

since neither system is accelerating vertically, we have
N_(table,1) - W_(Earth,1) = 0 &
N_(table,2) - W_(Earth,2) = 0
where 1 denotes the system that has box 1 (System 1) and 2 denotes the system that has box 2 (System 2).

Horizontally, for System 1, making the right positive, the two tension forces make possible the system's acceleration (from what I have currently):
2T_(rope,1) = m_(system 1)*a_1

For System 2, horizontally,
F - 2T_(rope,2) = m_(system 2)*a_2

@haruspex — I'm sorry, but it's still not obvious to me what I'm missing... Could you elaborate?
 

Attachments

  • Screen Shot 2018-08-12 at 3.43.18 PM.png
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JessicaHelena said:
it's still not obvious to me what I'm missing... Could you elaborate?
What is the rope attached to at its ends?
 
JessicaHelena said:
Horizontally, for System 1, making the right positive, the two tension forces make possible the system's acceleration (from what I have currently):
2T_(rope,1) = m_(system 1)*a_1

For System 2, horizontally,
F - 2T_(rope,2) = m_(system 2)*a_2
As haruspex points out. You are missing some forces. A rope will also exert a force on anything it is attached to. You cannot just ignore those forces.
 
@haruspex, @Orodruin

The rope is attached to box 1 on one end and the table on the other end. But I don't think the table is exerting any direct forces on System 1...
So would I get something like this (attached)?
 

Attachments

  • Screen Shot 2018-08-12 at 8.26.44 PM.png
    Screen Shot 2018-08-12 at 8.26.44 PM.png
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JessicaHelena said:
@haruspex, @Orodruin

The rope is attached to box 1 on one end and the table on the other end. But I don't think the table is exerting any direct forces on System 1...
So would I get something like this (attached)?
That looks right.
 
  • #10
Alright, thank you for all the help, everyone!
 

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