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## 60 thoughts on “Homework 27.A”

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You can check your answer for the shear force by checking that your shear force at point E equals the force P.

Agreed, this is a good way to check your work. Another way is that bending moment will return to zero at the end.

Do we need to consider the moment around the application points of the forces, or around A and D?

You should be sure to include Ax, Ay, and Dy

Bending moment diagram requires you to calculate BM at each cross section along the length of the beam. So it is better to write your moment equation about a fixed point like A. Hope this helps.

Hello Stefan,

We sum the moments around either A or B to eliminate one of thee reaction forces and solve for the other one then by summing the forces in the y-direction we can solve for the other reaction force

There are no moment at point A and D I think.

Bending moment diagram requires you to calculate BM at each cross section along the length of the beam. So it is better to write your moment equation about a fixed point like A. Hope this helps.

A is a pin joint, correct?

I believe it is. I had there a force in the Y direction, and a force in the X direction, but no moment around A

Hello Lucas,

Yes A is a pin joint, but since no horizontal forces are acting on the beam, thus it has only a vertical reaction force, so it seems like its acting like a roller support

I have it as a pin joint, however I think the x reaction is zero, since there are no forces in the x direction.

I think A as a pin joint. Since there is no force in the X-direction, I think A has only a Y-direction force.

I also had the reaction forces at A in the X and Y direction, but no moment present.

I think at A there's no reaction force in the x-direction, only in the y-direction

I believe you are correct that there is no moment at A because it is a pin joint; however, summing the forces in the x direction will only include Ax which must equal 0.

Yes, no reaction in X direction due to the impending loads only projecting in the y direction. No side loads.

Should I draw a free body diagram with shear forces shown at B, D and E? Or just B and E?

Hello Hyunseo Lee,

I am drawing a free body diagram for B,D, and E. They might only need two but I am drawing all three just in case.

I would also recommend drawing free body diagrams for B, D, and E.

Whenever we make a cut, will it always include a shear moment?

Yes, no matter where you cut, always put a positive shear force and a positive moment on the place you cut.

Wei-ting is right in that there will always be a positive shear force and a positive moment on the place you cut. Just remember that the side that you make your cut will influence the direction of the shear and moment. For example, if you make your cut and have the right face exposed, the shear force will be positive downward.

When making our V(x) and M(x) graphs, should keep them as open lines just on an xy plane or enclose the ends to the axies and shade in the area between the line and x-axis?

Hello Lukas,

According to the examples done, the V(x) graph is enclosed with shading and the M(x) graph is just the lines, without shading.

Is it assumed that the weight is negligible?

Yes, I believe that since the weight of the beam is not mentioned in the problem, we can assume that it is negligible.

When solving the problem, I assumed that the weight of the beam is negligible, as it isn't specified anywhere in the problem.

Yes, I believe weight is negligible as it is not mentioned in the problem

What does Vx and Mx stand for?

Vx is the sheer force and Mx is the bending moment

Yes, V represents shear force as a function of x, and M represents bending moment as a function of x also.

^^^

Where are there going to be moments present? Only at point E right?

Basically the moments occur at the ends of each segment which you can label as point P where the V force and Fn force are as well. It occurs at the ends where you cut off the segments. There shouldn't be a moment at Point E

Do I need to do a moment about A or D to solve the problem?

I believe when solving for your reaction forces, you use a moment about A in a global fbd so you're able to solve for the Dy reaction force. From there you can solve for the A reactions using sum of forces

How do we know the distance our cut is from our points?

You don't know, so you designate that distance as x. Therefore, the shear force and bending moment will both be functions of x. Additionally, x should always be measured from the same point for all cuts. That is, if you wanted to move left to right, you would have different x's for each segment, but they would all be measured from point A (for instance, for the second segment between B and D, x would be measured from A on the left to some point between B and D on the right. Therefore, d <= x <= 2d).

should x be in our answers for this homework

Yes, all shear forces and bending moments are functions of x.

How many free body diagrams should we draw?

You should have one for each segment in which the shear force and bending moment is different. So one for when 0 <=x<=d, a

second one for d <= x <= 2d, and a third one for 2d <= x <= 3d.

I had four in total, one for the entire system and and then three for each of the sections we are supposed to find.

Is there a moment at point A?

Yes, there will be reaction forces in x and y directions with moment at point A since it is a fixed support.

Sorry, my mistake, I think there is no moment at point A.

Just to clarify, there is no moment at point A because it is a pin joint as Chun-wei mentioned above.

Will the bending moment graph always return to zero if there is no reaction force at the end of the beam?

Correct, the bending moment graph will always return to zero. This is a good way to check if you did your calculations right.

When breaking up the sections of the beam, in the first section is there only the moment at the end of bending?

Yes. There's that moment and Ay that are needed to solve for the bending moment.

Why are the signs flipped for the V(x) calculation?

Will there be a moment about point A to calculate the bending moments?

No. You only need Ay at point A, since there's no moment and Ax equals zero, to solve for the bending moments.

Are there moments at A and D?

When Looking at Segment DE I assume we also need to account for the AD part in the moment and force equation right?

Yes, you need to include them to get the right answers

How do we find the shear force given the load is only applied at one point and not a distributed load?

Can we expect the line for M(x) to be continuous? I dont think V(x) has to be continuous.