I Seesaw problem with a man connected to a fan by a rope

[John Mcrain]

Men is at seesaw , connected with rope to beam which is conected to seesaw with joint,when fan is off ,seesaw is stay at brawn box.
When you turn ON fan,man and beam are in balance,can seesaw flip to right side?
(If beam is conect with fixed conect to seesaw then for sure it will flip to right side if fan thrust is enough...)

same question for this case,just joint from beam is at left side from seesaw pivot point..

If answer is yes...
To make seesaw flip to right side ,we must have some moment,but how we can transfer moment caused by fan if joint can't transfer moment only forces?

 

[A.T.]

man and beam are in balance,

This requires a moment balance at both: the man's heel and the joint. What does this imply for the moments by the man's gravity and the fan around the pivot?

 

[John Mcrain]

This requires a moment balance at both: the man's heel and the joint. What does this imply for the moments by the man's gravity and the fan around the pivot?

Man and beam is in balance mean that angle between man feet and seesaw and angle between beam where is fan and seesaw don't change,once when fan is turn ON.

let imagine like this; in initial position (fan turn off) seesaw is in balance.
What will happened with seesaw when turn ON fan?

 

[A.T.]

What will happened with seesaw when turn ON fan?

We have had this before:
https://www.physicsforums.com/threads/teeter-totter-tricky-problem.987781/post-6330131

 

[John Mcrain]

We have had this before:
https://www.physicsforums.com/threads/teeter-totter-tricky-problem.987781/post-6330131

So conclusion is that internal connection can not affect net moment?So I must look only at external forces when solving tasks ?

I don't get it how pitching moment can be done through joint?

 

[Lnewqban]

Men is at seesaw , connected with rope to beam which is conected to seesaw with joint,when fan is off ,seesaw is stay at brawn box.
When you turn ON fan,man and beam are in balance,can seesaw flip to right side?
(If beam is conect with fixed conect to seesaw then for sure it will flip to right side if fan thrust is enough...)
...
If answer is yes...
To make seesaw flip to right side ,we must have some moment,but how we can transfer moment caused by fan if joint can't transfer moment only forces?

If fan develops enough thrust, a moment will reach the man and the beam and everything that is solidly connected will flip.
The moment can't go from the beam to the ground because the pivot is unable to transfer it.

The pivot can transfer the thust force to the ground.
That induces a reaction force from the pivot that prevents the beam from sliding to the right and makes the flipping possible.

Same happens with the pivot that the braking wheels of taildragger airplanes are:
Please, see:
https://www.sandiegoskytours.com/flyers-blog/2018/7/16/the-good-the-bad-and-the-ugly-the-ugly

 

[A.T.]

If fan develops enough thrust, a moment will reach the man and the beam and everything that is solidly connected will flip.

But they are not solidly connected The system has two internal joints (man's heel & fan mast base) that must be stabilized. ´With the heel placed as shown in the image (left of the pivot), the man cannot stabilize the fan mast base, if the fan thrust is large enough to flip the whole thing.

 

[Lnewqban]

But they are not solidly connected The system has two internal joints (man's heel & fan mast base) that must be stabilized. ´With the heel placed as shown in the image (left of the pivot), the man cannot stabilize the fan mast base, if the fan thrust is large enough to flip the whole thing.

Yes, your statement is correct and mine was not.

To OP:
Please disregard my previous post as I was mistakenly referring to the ground-beam pivot, as well as assuming that the man could hold mast in perpendicular position respect to beam while increasing fan thrust.

 

[Lnewqban]

... I don't get it how pitching moment can be done through joint?

The mast, the rope, the man's body and the beam form a planar four-bar linkage.
Please, see:
https://en.m.wikipedia.org/wiki/Four-bar_linkage

The thust force of the fan can't make the mast rotate around its pivot to the beam only because the clock-wise moment that fan and mast induce is exactly compensated by the counter-clockwise moment that the weight of man and feet-to-mast distance induce (as per your original post).

As you relocate the ground-to-beam pivot towards the right, the magnitude of the clockwise moment remains the same, but the magnitude of the counter-clockwise moment increases, as the feet-to-ground pivot increases.

While the above happens, our four-link linkage remains stable because its balance of moments about the mast-to-beam pivot remains the same.