Weight Scale Logic: Solving Problems without Calculation

[johnrot]

[mentor note: edited for clarity]

The man weighs 100kg.

All constructions, ropes, universal joints, rollers, fans, etc are massless.

Friction between weight scale and feet or construction is enough to hold side forces...

The red line is the rope.

What will the weight scale show? <100kg ,100kg or >100kg...

All problems can be solved with logic, no need for calculation.

Do you agree with the results shown below each diagram?

 

[etotheipi]

This man is trying very hard to avoid the gym

 

[kuruman]

Are we to assume that the man is not accelerating in any of the cases?

If the man is in static equilibrium in all cases, I disagree with your results. Please provide justification for each one. Hint: You need to recognize the same free body diagram (FBD) for the man when you see it.

 

[A.T.]

[mentor note: edited for clarity]

The man weighs 100kg.

All constructions, ropes, universal joints, rollers, fans, etc are massless.

Friction between weight scale and feet or construction is enough to hold side forces...

The red line is the rope.

What will the weight scale show? <100kg ,100kg or >100kg...

All problems can be solved with logic, no need for calculation.

Do you agree with the results shown below each diagram?https://www.physicsforums.com/attachments/261243

Is thrust the same in all cases? Is the man's center of mass the same with rope (bottom center) as without (bottom left)?

 

[johnrot]

Is thrust the same in all cases? Is the man's center of mass the same with rope (bottom center) as without (bottom left)?

question for thrust is irrelevant,look each picture separately, everything what you need is on picture...
rope is connect at men c.g.

I add numbers for each case so we can easier discuss..

 

[phinds]

I add numbers for each case so we can easier discuss..

And yet you did not provide the requested discussion (made in post #3) of your conclusions.

 

[A.T.]

question for thrust is irrelevant

So your answers include the case that thrust is the same in case 9) and 10) ?

rope is connect at men c.g.

That wasn't my question. Is the position of the c.g the same in case 9) and 10) ?

 

[Cutter Ketch]

As nobody has really said, I will say 1-9 look right to me. Does anybody have a problem with 1-9?

 

[phinds]

As nobody has really said, I will say 1-9 look right to me. Does anybody have a problem with 1-9?

You continue to avoid the request to justify your answers and show your thinking on each.

 

[kuruman]

As nobody has really said, I will say 1-9 look right to me. Does anybody have a problem with 1-9?

I do (maybe). I have no problem with 1-5. But let's wait until we hear from OP.

 

[johnrot]

So your answers include the case that thrust is the same in case 9) and 10) ?That wasn't my question. Is the position of the c.g the same in case 9) and 10) ?

I honestly don't understand the question
Dont you see that men is lean to the left on case 10)??

 

[jim mcnamara]

@johnrot if you do not show your attempts for 1-9 then we will have to lock the thread. PF is here to help, we are not a homework service. Please post your attempts, this could be an interesting thread - if you can follow the rules.

 

[johnrot]

@johnrot if you do not show your attempts for 1-9 then we will have to lock the thread. PF is here to help, we are not a homework service. Please post your attempts, this could be an interesting thread - if you can follow the rules.

What I must explain 1 and 9 case,or from 1 to 9?
what cases are "problem"?

(I am not here for searching help,I post this to see how different people think..)

 

[jim mcnamara]

Then this belongs in general discussion, not homework. I"m moving it for you.

 

[phinds]

(I am not here for searching help,I post this to see how different people think..)

So you want us to say how we think, but you do not want to say how you think. Not an attitude that is likely to give people much incentive to answer.

 

[johnrot]

So you want us to say how we think, but you do not want to say how you think. Not an attitude that is likely to give people much incentive to answer.

(my answers are already there,below every case..)
What case I must explain?

 

[phinds]

What case I must explain?

Any case that you want someone else to comment on. Why do you continue, after numerous requests, to refuse to state your reasoning for each of the cases you have presented? You have indicated that you think you know the answer to the first 9 but we have no idea WHY you think that.

I can't speak for others, but one more refusal and I'm outta here. I've been wanting to attempt to help but I'm getting bored with asking you for your logic.

 

[DrClaude]

Thread closed pending moderation.

This thread is going to reopen to allow you to provide an explanation why you think each case is as you noted it in the picture.

By the way, the fact that this is not homework is irrelevant to the fact that it looks like homework, hence the PhysicsForums rules for homework apply.

 

[johnrot]

1) men has 100kg so weight scale show 100kg.
2)rope is 100% horizontal,so there is no vertical component ,so weight stay the same ,100kg
3)rope is angled upward,now we have vertical component which will subtract some of men weight,so <100kg
4)rope is angled downward,now we have vertical component which will add some weight, >100kg
5) same as case 3)
6)men is lean back to the left,massless construction have rigid conection,if there is no rope which conect construction with ground,men and construction will fall down to the left (on ground),so we must put this rope ,let say this rope has tension T. weight scale will show 100kg+T so >100kg
7)fan thurst and rope tension don't have vertical components so weight stay the same,100kg
8)rope which conect men and fan-mast is angled upward so some of men weight is transferd to universal joint (u.j.), so if we don't have rope at right end of construction,it will fall down to the ground..lets say rope have tension T,

then weight scale will show 100kg-T so <100kg
9)if fan is turn OFF weight scale will show 100kg,fan is conect to construction with rigid conection so when fan is turn ON,then construction will try to rotate in clockwise direction about u.j. ,because of this, weight scale will show <100kg,

(if fan thrust is large enough to lift construction completely from weight scale(not case in my diagram) then weight scale will show 0kg..)
10)men and fan-mast is in balance,rope is 100%horizontal so we don't have veritcal component which will transfer some of men weight to the u.j. at fan-mast.

fan-mast is conect with u.j. to construction that mean that fan thrust can not transfer moment to the construction because universal joint can not transfer moment only forces,so we have thrust from fan in level at u.j. this thrust act below pivot point of construction which mean it will try to rotate construction in conter clockwise direction,so construction is pushing more to the weight scale...so >100kg
11)men lean back to the left on rigid massless construction,if fun is turn OFF ,men and costruction will fall down to the left,so fan must be turn ON to stop this rotation,fant thrust is 100% horizontal so no vertical components,weight scale show 100kg

 

[A.T.]

View attachment 26125910)men and fan-mast is in balance,rope is 100%horizontal so we don't have veritcal component which will transfer some of men weight to the u.j. at fan-mast.

fan-mast is conect with u.j. to construction that mean that fan thrust can not transfer moment to the construction because universal joint can not transfer moment only forces,so we have thrust from fan in level at u.j. this thrust act below pivot point of construction which mean it will try to rotate construction in conter clockwise direction,so construction is pushing more to the weight scale...so >100kg

Considering external moments:

If the man in 10) moves his c.g. to the left in order to lean, then that increases the CCW moment by gravity around the pivot. And this increase of CCW moment is always greater than the CW moment by thrust around the pivot (as long as the pivot is higher than the feet & mast u.j.).

So I'm fine with 10) if the man's c.g. is not in the same position as in 9).

 

[johnrot]

Considering external moments:

If the man in 10) moves his c.g. to the left in order to lean, then that increases the CCW moment by gravity around the pivot. And this increase of CCW moment is always greater than the CW moment by thrust around the pivot (as long as the pivot is higher than the feet & mast u.j.).

So I'm fine with 10) if the man's c.g. is not in the same position as in 9).

If men will lean back in case 9) ,now we have this situation:

in case 10) if men and fan-mast is in balance ,whatever you do with men lean angle,rope angle and thrust you can never rotate all construction in clockwise direction about pivot point...
men lean angle,rope angle determine magnitude of thrust,if men is lean back more or if rope is at more vertical angle you can ahcieve bigger thrust...

 

[jbriggs444]

in case 10) if men and fan-mast is in balance ,whatever you do with men lean angle,rope angle and thrust you can never rotate all construction in clockwise direction about pivot point...
men lean angle,rope angle determine magnitude of thrust,if men is lean back more or if rope is at more vertical angle you can ahcieve bigger thrust...

I found it most helpful to start by considering torques on the teeter board (the rigid assembly consisting of the lower board, the upper board and the vertical attachment between them) about a pivot point at the man's feet.

This is handy because the weight scale reading is the unknown we are working to determine. Choosing an axis at the location of an unknown force is almost always a good idea.

 

[berkeman]

FYI, the OP has left the building. Thank you everybody for trying to decode his problem statements and help him out. Thread is now closed.