Understanding Moment Direction: Analysis and Solution for Homework

[chetzread]

Homework Statement

why does the moment -400Nm only act at region B and C?

Why it doesn't act from A to C?

http://imgur.com/a/5tfCg

Homework Equations

The Attempt at a Solution

As we can see from figure, there's moment 400Nm on the beam at B at the top part and also the below part...

 

[chetzread]

notes here

 

[chetzread]

is it wrong?

 

[SteamKing]

Homework Statement

why does the moment -400Nm only act at region B and C?

Why it doesn't act from A to C?

http://imgur.com/a/5tfCg

Homework Equations

The Attempt at a Solution

As we can see from figure, there's moment 400Nm on the beam at B at the top part and also the below part...

The moment at B is called a couple. It acts on the beam as a whole. It's applied at point B, so naturally it acts there. The beam is fixed at point C, so a moment reaction will develop there to keep the beam in static equilibrium. This is why the moment diagram looks the way it does.

 

[chetzread]

The moment at B is called a couple. It acts on the beam as a whole. It's applied at point B, so naturally it acts there. The beam is fixed at point C, so a moment reaction will develop there to keep the beam in static equilibrium. This is why the moment diagram looks the way it does.

so, the moment 400Nm at B has anticlokwise direction, it cause the beam at B to become concave downwards, at c, the moment is clockwise , so , the beam at c will also concave downward , so, at both B and C the moment is negative, am i right?
Thus, moment anticlokwise = moment clockwise , beam in equlibrium?

 

[SteamKing]

so, the moment 400Nm at B has anticlokwise direction, it cause the beam at B to become concave downwards, at c, the moment is clockwise , so , the beam at c will also concave downward , so, at both B and C the moment is negative, am i right?
Thus, moment anticlokwise = moment clockwise , beam in equlibrium?

At B, the couple is CCW and has a positive magnitude.
At C, the reaction must be CW, and the moment there will have a negative magnitude, to ensure that there is no net moment acting on the beam.

 

[chetzread]

At B, the couple is CCW and has a positive magnitude.
At C, the reaction must be CW, and the moment there will have a negative magnitude, to ensure that there is no net moment acting on the beam.

If the moment at B is positive, then why it's - 400Nm oat B on the graph?

 

[SteamKing]

If the moment at B is positive, then why it's - 400Nm oat B on the graph?

It's not -400 N-m at B. The diagram is indicating that the reaction moment at C is -400 N-m, so that the net moment becomes zero.

 

[chetzread]

The

It's not -400 N-m at B. The diagram is indicating that the reaction moment at C is -400 N-m, so that the net moment becomes zero.

Then it's positive 400Nm at B?
Why the author drew the line as negative negative 400Nm from B to C?

 

[SteamKing]

The

Then it's positive 400Nm at B?
Why the author drew the line as negative negative 400Nm from B to C?

I think you're misinterpreting the diagram.

After all, there is a lot of additional information being presented here, and some compromises must be made on presentation.

 

[chetzread]

I think you're misinterpreting the diagram.

After all, there is a lot of additional information being presented here, and some compromises must be made on
presentation.

Can you whether diagram is negative 400Nm from B to C? Why not positive 400Nm at B?

 

[chetzread]

I think you're misinterpreting the diagram.

After all, there is a lot of additional information being presented here, and some compromises must be made on
presentation.

Why can't I interprete the moment causes the beam to concave downward as negative? I ran that I was told this theory earlier in the forum...

 

[SteamKing]

Can you whether diagram is negative 400Nm from B to C? Why not positive 400Nm at B?

The magnitude of the couple is +400 N-m at B.

In order for the beam to remain in equilibrium, the reactive couple at C must be -400 N-m, which is what the diagram indicates to me.

 

[chetzread]

The magnitude of the couple is +400 N-m at B.

In order for the beam to remain in equilibrium, the reactive couple at C must be -400 N-m, which is what the diagram
indicates to me.

Since you said that the moment at B is positive 400Nm, then why the graph shouldn't change from positive 400Nm to negative 400Nm from B to C?

 

[SteamKing]

Since you said that the moment at B is positive 400Nm, then why the graph shouldn't change from positive 400Nm to negative 400Nm from B to C?

If you look at the moment plot at C, it clearly indicates that the reactive moment there is -400 N-m, which, when added to the moment at B of +400 N-m, equals zero net moment.

Your interpretation of what the plot would look like would require a net change in moment of -800 N-m, which is not supported by the loading of the beam.

 

[chetzread]

So, we draw the graph from C to B?
That's very the graph stop at B - 400Nm +400Nm =0?

 

[chetzread]

So, we draw the graph from C to B?
That's very the graph stop at B - 400Nm +400Nm =0?

 

[SteamKing]

So, we draw the graph from C to B?
That's very the graph stop at B - 400Nm +400Nm =0?

It's not clear what you mean here.

The moment diagram doesn't plot discrete values of BM in isolation, like you apparently want to do.

At point B, the moment goes from 0 to +400 N-m, which is why there is a jump at this location.

The moment remains constant at +400 N-m from point B to point C, where the reactive moment of -400 N-m is added to the +400 N-m, to bring the net moment back to zero.

You should review how shear force and bending moment values are plotted for regular beam problems.

 

[chetzread]

It's not clear what you mean here.

The moment diagram doesn't plot discrete values of BM in isolation, like you apparently want to do.At point B, the moment goes from 0 to +400 N-m, which is why there is a jump at this
location.The moment remains constant at +400 N-m from point B to point C, where the reactive
moment of -400 N-m is added to the +400 N-m, to bring the net moment back to zero.


You should review how shear force and
bending moment values are plotted for regular beam problems.

It's not clear what you mean here.The moment diagram doesn't plot discrete values of BM in isolation, like you apparently want to doAt point B, the moment goes from 0 to +400 N-m, which is why there is a jump at this

location.

The moment remains constant at +400 N-m from point B to point C, where the reactive

moment of -400 N-m is added to the +400 N-m, to bring the net moment back to zero.

You should review how shear force and
bending moment values are plotted for
regular beam problems.

Changing from positive 400Nm to 0 from B to C, why shouldn't the graph drawn at the positive side?
I mean in the figure, the author indicated it as negative 400Nm and at C, it become 0
Why shouldn't the graph drawn from positive 400Nm to 0 from B to drop 'directly' to become 0 at C?

 

[SteamKing]

Changing from positive 400Nm to 0 from B to C, why shouldn't the graph drawn at the positive side?

The "positive" side is the side which is shown by the author on the diagram. Certain allowances must be made so that the two diagrams don't overlap, which would make things even more confusing.

I mean in the figure, the author indicated it as negative 400Nm and at C, it become 0
Why shouldn't the graph drawn from positive 400Nm to 0 from B to drop 'directly' to become 0 at C?

I think I have explained this diagram enough. If you want to plot a different diagram, that is your prerogative.

 

[chetzread]

The "positive" side is the side which is shown by the author on the diagram. Certain allowances must be made so that the two diagrams don't overlap, which would make things even more confusing.I think I have explained this diagram enough. If you want to plot a different diagram, that is
your prerogative.

But, we can see, the author drew it under the graph, which is - 400Nm, how could it be positive?

 

[SteamKing]

But, we can see, the author drew it under the graph, which is - 400Nm, how could it be positive?

You're taking things too literally. I bet when you were a kid, you always colored within the lines.

 

[chetzread]

You're taking things too literally. I bet when you were a kid, you always colored within the lines.

I am still confused, can you explain in another way?

 

[SteamKing]

I am still confused, can you explain in another way?

Nope.

 

[chetzread]

If you look at the moment plot at C, it clearly indicates that the reactive moment there is -400 N-m, which, when added to the moment at B of +400 N-m, equals zero net moment.

Your interpretation of what the plot would look like would require a net change in moment of -800 N-m, which is not supported by the loading of the beam.

what i mean is we should draw positive 400Nm throughout B and C(since you said that at B, the moment is positive 400Nm), then specifically at C. the moment drop sharply to 0 ..Is it wrong to do so?

 

[SteamKing]

what i mean is we should draw positive 400Nm throughout B and C(since you said that at B, the moment is positive 400Nm), then specifically at C. the moment drop sharply to 0 ..Is it wrong to do so?

No, it isn't.

 

[chetzread]

No, it isn't.

Can i do so?
No , it isn't means i can do so?

 

[chetzread]

No, it isn't.

How is this case different from the previous case?
https://www.physicsforums.com/threads/moment-diagram.878559/#post-5520349
i was told that to keep the beam in equilibrium, the moment has to be applied at the another end (wall)in opposite direction
But, in this question, i have 2 ends, namely A and B, but both point doesn't restrict moment, am i right?
In the previous therad, it's wall, so it can restrict moment...But , how about this case?

 

[SteamKing]

How is this case different from the previous case?
https://www.physicsforums.com/threads/moment-diagram.878559/#post-5520349
i was told that to keep the beam in equilibrium, the moment has to be applied at the another end (wall)in opposite direction
But, in this question, i have 2 ends, namely A and B, but both point doesn't restrict moment, am i right?
In the previous therad, it's wall, so it can restrict moment...But , how about this case?

It's not clear what you are saying.

By now, you should be able to take a beam, create a free body diagram for it, and then write the equations of static equilibrium.

You shouldn't be relying on "he said, she said." for analyzing these problems.

 

[chetzread]

It's not clear what you are saying.

By now, you should be able to take a beam, create a free body diagram for it, and then write the equations of static equilibrium.You shouldn't be relying on "he said, she
said." for analyzing these problems.

OK, why the moment isvre

It's not clear what you are saying.

By now, you should be able to take a beam, create a free body diagram for it, and then write the equations of static equilibrium.

You shouldn't be relying on "he said, she said." for analyzing these problems.

Why the moment is restricted about A, but not at C?

 

[SteamKing]

Why the moment is restricted about A, but not at C?

I don't know what you mean by "restricted about A, but not at C".

The beam is fixed at C, so a reactive moment is supported there. The beam is free at A.

 

[chetzread]

I don't know what you mean by "restricted about A, but not at C".

The beam is fixed at C, so a reactive moment is supported there. The beam is free at A.

https://www.physicsforums.com/threads/moment-diagram.878559/#post-5520349
I am sorry for the previous post...
I mean why in this thread, the moment of the beam is not restricted about B , but is restricted about A ?
I

 

[SteamKing]

https://www.physicsforums.com/threads/moment-diagram.878559/#post-5520349
I am sorry for the previous post...
I mean why in this thread, the moment of the beam is not restricted about B , but is restricted about A ?
I

I still don't know what you mean by the moment being "restricted" about A. There is no moment about A.

 

[chetzread]

I still don't know what you mean by the moment being "restricted" about A. There is no moment about A.

As you can see, the moment 400Nm graph is drawn from A to the point where moment 400Nm act, so there's moment restriction about point A, right? Why there's no moment restriction about point B?
https://www.physicsforums.com/threads/moment-diagram.878559/#post-5520349

 

[SteamKing]

As you can see, the moment 400Nm graph is drawn from A to the point where moment 400Nm act, so there's moment restriction about point A, right? Why there's no moment restriction about point B?
https://www.physicsforums.com/threads/moment-diagram.878559/#post-5520349

There is no moment applied at point A, so there won't be one showing up on any moment diagram.

There is a moment applied at point B, which is why the moment diagram starts there.