Moment at Two Ends of Beam: Understanding Variations and Direction

[chetzread]

Homework Statement

in the first diagram , i notice that the moment is constant throughout the beam...

why in the second notes, for the moment 20kNm at 2 ends of beam, why the moment varies from -20kNm to 0 from one end to another end?

Homework Equations

The Attempt at a Solution

Which is correct? Which is wrong?
i'm confused...
another thing that i noticed is why no matter moment is clockwise or anticlockwise, the moment is always -20kNm??(as we can see ,moment is anticlockwise on the left , clockwise on the right...)

 

[SteamKing]

Homework Statement

in the first diagram , i notice that the moment is constant throughout the beam...

why in the second notes, for the moment 20kNm at 2 ends of beam, why the moment varies from -20kNm to 0 from one end to another end?

Homework Equations

The Attempt at a Solution

Which is correct? Which is wrong?
i'm confused...
another thing that i noticed is why no matter moment is clockwise or anticlockwise, the moment is always -20kNm??(as we can see ,moment is anticlockwise on the left , clockwise on the right...)

You can't compare beams willy-nilly. The support conditions influence how the forces and moments distribute within the beam.

In the first case, you have a cantilever beam, with the left end fixed and a couple applied at the opposite end. In order to maintain static equilibrium, there will be a constant moment present along the length of the beam, as shown in Fig. (b). The fixed end of the cantilever has a reactive couple, which combines with the applied couple to make a zero net moment for the beam.

In the other two beam cases, the beam is pinned at each end and a couple of magnitude 20 kN-m is also applied at each end. Pinned connections cannot support a moment, so the moment diagram shows the magnitude of each couple where it is applied and a zero moment at the opposite end of the beam, with a linear slope in between.

 

[chetzread]

You can't compare beams willy-nilly. The support conditions influence how the forces and moments distribute within the beam.

In the first case, you have a cantilever beam, with the left end fixed and a couple applied at the opposite end. In order to maintain static equilibrium, there will be a constant moment present along the length of the beam, as shown in Fig. (b). The fixed end of the cantilever has a reactive couple, which combines with the applied couple to make a zero net moment for the beam.

In the other two beam cases, the beam is pinned at each end and a couple of magnitude 20 kN-m is also applied at each end. Pinned connections cannot support a moment, so the moment diagram shows the magnitude of each couple where it is applied and a zero moment at the opposite end of the beam, with a linear slope in between.

why moment at the opposite end of the beam will be 0 ? why not positive 20kNm ?

 

[SteamKing]

why moment at the opposite end of the beam will be 0 ? why not positive 20kNm ?

I explained that in my post.

If you are not going to read the replies to your questions, what are you doing on this site?

 

[chetzread]

You can't compare beams willy-nilly. The support conditions influence how the forces and moments distribute within the beam.

In the first case, you have a cantilever beam, with the left end fixed and a couple applied at the opposite end. In order to maintain static equilibrium, there will be a constant moment present along the length of the beam, as shown in Fig. (b). The fixed end of the cantilever has a reactive couple, which combines with the applied couple to make a zero net moment for the beam.

In the other two beam cases, the beam is pinned at each end and a couple of magnitude 20 kN-m is also applied at each end. Pinned connections cannot support a moment, so the moment diagram shows the magnitude of each couple where it is applied and a zero moment at the opposite end of the beam, with a linear slope in between.

why not positive 20kNm ? why it's 0 ? we can see , the moment 20kNm at the left ( anticlockwise) is opposite to the moment on the right (clockwise)