How should forces be drawn on a ladder? Considering the cross-members

[bolzano95]

< Mentor Note -- thread moved to HH from the technical physics forums, so no HH Template is shown >

In the attached file we can see simplified ladder. I am interested on which side (right or left) the forces are correctly drawn.
I would draw forces as the right side shows.
So which one is right?

 

[berkeman]

The right side drawing appears clearly wrong, but can you say more about the force that is applied? Are you pushing down on the top of the ladder, or pulling up? Is the ladder constrained to stay in that shape somehow, or could it collapse given enough force applied?

 

[bolzano95]

There is no additional information given in the problem statement. It only says to draw the forces on the top of a ladder.
berkerman - why do you say the right side wrong? I am interested in your argument.
Without question there are horizontal forces which point in right direction - I am interested in vertical components. Do they point up or down?

 

[berkeman]

Please remember to post schoolwork questions in the Homework Help forums, and to fill out the Template you are provided there. I'll move this thread to the schoolwork forums now for you.

So the problem statement says there is a force straight down on the top of the ladder, and asks you to draw the FBD including the reaction forces? Why are you drawing horizontal force vectors -- are those meant to be reaction forces that cancel out?

 

[bolzano95]

berkman,
The problem does not state anything about a force applied up or down. It states: Draw forces on top of the ladder.
I presumed the branches are weightless.
Yes, those are reaction forces that cancel each other.
I'm tying to determine in which direction does the normal force of each branch of the ladder point.

 

[Dale]

So which one is right?

It doesn't really matter. Just pick one and work the problem. If you get a negative value for F then you know you drew it in the wrong direction, but at that point you don't even need to go back and fix the drawing. It is perfectly acceptable to have a negative value.

 

[kuruman]

I'm tying to determine in which direction does the normal force of each branch of the ladder point.

Then draw each branch separately and not the two together.

 

[berkeman]

Also @bolzano95 -- you should be sure to include the cross-members that are part of a folding ladder like that. The FBD for each side of the ladder will not be complete without including the forces in each cross-member that forms a triangle to limit how far the two outside ladder pieces can fold out...

http://www.wrestlingtrader.co.uk/wp-content/uploads/2016/07/Black-Ladder.jpg

 

[haruspex]

you should be sure to include the cross-members

For the complete FBDs, yes, but I gather that at this point the question is purely how to draw the forces at the top joint. Besides, the structure can stand without cross members given sufficient friction at the ground.
Kuruman made the most important point: draw each leg of the ladder separately.
You can assume that the applied vertical load is shared equally. To achieve a balance, you will need to represent a force along the leg.