# Which forces am I missing in the FBDs?

• JessicaHelena
In summary, the massless rope is attached to two blocks with pulleys, and a force of magnitude F is applied to the second block. 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.
JessicaHelena

## 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

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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

• Screen Shot 2018-08-11 at 7.28.02 PM.png
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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
6.2 KB · Views: 412
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.

Alright, thank you for all the help, everyone!

## 1. What is an FBD?

An FBD, or Free Body Diagram, is a visual representation of all the forces acting on an object. It helps to simplify and visualize complex systems and analyze the motion of an object.

## 2. Why is it important to consider all the forces in an FBD?

In order to accurately predict the motion of an object, it is crucial to consider all the forces acting on it. Neglecting even one force can result in incorrect calculations and predictions.

## 3. What are some common forces that are often missing in FBDs?

Some common forces that are often missing in FBDs include frictional forces, tension forces, and normal forces. Air resistance and buoyant forces may also be overlooked in certain situations.

## 4. How can I identify the forces that are missing in my FBD?

One way to identify missing forces in an FBD is to carefully analyze the scenario and think about all the possible forces that could be acting on the object. You can also refer to the laws of motion and principles of physics to determine which forces may be present.

## 5. What are the consequences of not including all forces in an FBD?

Not including all forces in an FBD can lead to incorrect predictions of an object's motion. This can result in errors in engineering designs, safety hazards, and inaccurate scientific conclusions. It is important to always consider all forces in an FBD to ensure accurate analysis and predictions.

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