# Homework 24.A We are encouraging you to communicate with us and with your colleagues in the class through the threaded discussions on the course blog. If you have questions on this homework, please ask here.

## 49 thoughts on “Homework 24.A”

1. Alexander Luis Yeverino says:

Should there be any forces acting at C such as Cx or Cy as force couples?

1. Robert Christian Corridan says:

Yeah, there is a pin at C connecting the two bodies together. Because of that there exists 'two' reaction forces at C Cx and Cy. It should be noted that Cx and Cy of the weird half circle thing will be in the opposite direction of the Cx and Cy forces acting on the backwards J bar.

1. Hayden James Lang says:

How do you know when it is best to but the reaction forces for C in the negative or positive directions for each of the separated free body diagrams?

2. Aileen C Ryan says:

There are forces acting at C. When drawing both members in one FBD, you don't show them. When drawing separate diagrams, you have to include them.

3. Shivani Pulugura says:

Yes there are two forces acting on C and drawing the FBD of them will allow you to see that

2. Elliot Porter Stockwell says:

If member AC is a two force member, why can't we assume that the reaction force is along the line of action from point A to point C?

1. Gen Li says:

I think you can assume that in this case. Since AC is a two force member, you can combine the reaction forces at A and C into two collinear forces in opposite directions. Then the y component reaction force at A would just be the force along the line of action from A to C, and there would be no x reaction force.

1. Caitlyn Prill says:

I am confused on why there isn't a reaction force Ax but there is a reaction force Bx. To me these situations look very similar.

1. Ritvik Punjari says:

Caitlyn,
There is certainly a reaction force Ax, however if you choose to treat member AC as a two-force member (which is what the explanation is regarding in this thread), then you wouldn't have to consider the reaction forces at A when drawing the individual free-body diagrams for AC and BD. Note that you should definitely have an Ax reaction force in your overall FBD.

I hope this helps!

3. Kate Elizabeth Wilson says:

I am confused on how to set up the forces on AC (the two-force member). If you don't need x and y components for points A and C, how would you simplify that?

1. Wei-ting Pan says:

We do need x and y components for points A and C. In C-shaped part, if you set up a moment equation at points A and C, you will get Ax and Cx equal to 0 because AC is a two forces member. The next thing we need to do is to set up a moment equation and sum of force for this system. Since you already know the value of Ax, you are able to get the values of Ay, Bx, and By.

4. Dan says:

How should we break down the system into 2 parts?

1. Jiyoon says:

We can divide the system into just the C-shaped part and the other part without the C-shape.

2. Kristen Nicole Kuhrt says:

Hi Dan,
I would break the system into 2 parts by detaching the C shaped structure from the L shaped structure. You can then identify the forces that act on each of the separate structures.

3. Rithwik Raghuram says:

We break the system into the C component and the L component.

5. Yash Gaurav Mathur says:

How do I identify the two force members here?

1. TA - Wang, Xiaokang says:

there is an object that interacts with the rest of the world at ONLY 2 points (weight is negligible).
AC, 2 points. BCD, 3 points.

2. Taylor Marie Brown says:

A two force member is a component that only has two points of load. In the diagram, we can see that AC only has two points of load, since it only has two pin joint connections

6. Mitchell Lee Kisaberth says:

Since there will be moment reactions at A and B, how do we write our answer without R in the answer for these moments?

1. Mitchell Lee Kisaberth says:

Nevermind. Edit: Pin joints are at A and B, meaning there are no reactions.

2. Alexander Luis Yeverino says:

Be careful, I believe there are also reactions Cx and Cy acting on the system.

7. David says:

I am wondering if we need to use any moment equation to solve for the reactions at A? don't think we do

1. TA - Wang, Xiaokang says:

I think you already know how to do after the tutorial room.

Just for others, Yes, you need moment equation.

1. Hyunseo Lee says:

Would this moment equation be about point A? Or do we have to use multiple momnet equations? I am just confused on how to set up moment equation for each parts

2. Colt Parker says:

In most problems, I have found that using moments is very helpful because it allows for you to disregard forces in the FBD at the point you are choosing to sum your moments around. For example, when I solved 24.A, I used sum of moments at B for the member DCB. This allows us to find Cy in terms of P right away since sum of moments isolates Cy and P into the same equation. I hope this helps!

8. Pedro Ornia Reyes says:

Why is AC a two-force member? I don't understand how it has no reaction in the x direction at A.

1. Jack Davey Petruccione says:

I took this to be because joint C is a pin. When you push on point D, imagine how things would start to move or flex. As such, think about what impending motion the system would have if the pin joint at A were to be released. A would feel no reaction in the X because this impending motion is vertical. Think about C as being tangent to the constraint circle through which the assembly attached at B can move.

9. Mehal Sharma says:

Is there a reaction force in the x-direction at C?

1. Alexander Lee Dixon says:

Nope, it's because AC is a two force member, so in this case there will only be a y-reaction force.

2. Owen says:

I agree, this can also be proven by taking the moment about point A, which would result in Cx = 0. This would also imply that Ax = 0, which creates the two force member.

1. Evan Paul Lee says:

Is there a good way to determine that C is a two-force member by inspection? Or is it best way to quickly take the moment or sum forces on the member?

1. Kathryn Zhi Shang Albean says:

Hi Evan, yes AC is a two member force because there are only two force acting on that member, at point C and at point A. However, the "L" shape member has three forces acting on it, P at D, C, and at point B. Because there are more than two forces acting it is not a two force member. I hope this helps.

2. MarcAntonio Samuel Aragona says:

Since there are only two forces acting on the member, we can easily assume that this would be a two-force member.

10. Victoria Pompeo says:

Does the shape of the bar AC alter the problem in anyway? I understand we have to utilize radius instead of distance, but is there any other factors we need to consider when solving this problem?

1. Taylor Marie Brown says:

Hi Victoria! The shape of the bar does not affect the problem in any way. However, the fact that AC is a 2-force member is the most important part of the member

2. Noah Michael Boursier says:

Because the members are massless in our problems it doesn't matter what the shape of the member is, only the forces on it. I want to draw one as a giant mass of spaghetti.

11. Haley Jean Etheridge says:

Is the vertical force at C the same as the vertical force at A?

1. Morgan Kinley Smith says:

They should be going in opposite directions, so one will be pointing up and the other will be going down for the C shaped member.

2. Kareem Yasser AlDohaim says:

Member AC is a two force member, so the reactions at A and C are equal but opposite since they act along the same line.

12. Jaden Adam Kent says:

Is the AC arc a double force member? like can we treat it as a truss component?

1. Nicholas Michael Paulter says:

Yes, AC is a two force member. Therefore you can treat it as a truss component with Fac acting up and down at either ends of the arc. Using this, moment, and the sum of the forces, you should be able to find the reaction force you are looking for. Hope this helps!

2. Taylor Marie Brown says:

Yes, AC is a two-force member. This means you are able to treat it similarly to a truss, which will allow you to find the reactions at A

13. Shreyas Varathan says:

Can anyone explain why AC is a two-force member?

1. Diego Andres Siguenza Alvarado says:

Hi Shreyas. AC is a two-force member because it would not be in equilibrium otherwise (you can try to do the sum of forces and sum of moments to convince yourself). And these forces shall have the same magnitude and same action line, but opposite direction. In general, a rigid body with applied forces only in its extremes is considered a two-force member.

1. Nicholas Adam Barron says:

what he said ^

2. Colt Parker says:

While AC has reaction forces Ax, Ay, Cx, and Cy, the reaction forces can also be written as two vectors each with two components. These vectors can be written as

F(at A) = Ax i + Ay j
and
F(at C) = Cx i + Cy j

The important thing to see is that the reaction forces can be written as two individual forces and are the only forces on member AC thus making it a two-force member.

14. Zachary J Hansen says:

When drawing the initial FBD, how do we know which way to draw reaction forces. Sometimes this changes the signs I have in finals answers.

1. Michael Benjamin Metzner says:

When drawing the reactions, they can be any way you like as long as their counterpart on the other FBD is equal in magnitude and opposite in direction. For example, if you have two members joined by a pin A, the reactions on one FBD will be in the positive x and y-directions and the reactions on the other FBD will be in the negative x and y-directions.

15. Hunter Thompson says:

I am confused on how to find Ay and By if there is no outside force acting on the structure in the y direction. Do I need to take the moment of part of the structure to solve for them?