Statics Mechanics: Looking for the flaw in my logic

In summary, the student is trying to solve a problem in statics where they are given three cables with different tensional forces, and are not given any information about the pins attaching the pulleys to the rope. If there are no pins attaching the pulleys to the rope, they will slide down and the whole thing will collapse.
  • #1
Femme_physics
Gold Member
2,550
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Yep...back to old statics. Not because I have a test (already passed it in flying colours) just for fun seeing what I can solve and can't...now I ran into this:

Homework Statement


http://img705.imageshack.us/img705/1533/pulleyf.jpg [Broken]

Statics:

The weight is 50kg. Find force P to hold it in equilibrium.

The Attempt at a Solution


http://img18.imageshack.us/img18/5787/pulleya.jpg [Broken]Somehow I got the result in minus, trying to find where is the logic in this. Or, otherwise, where is the flaw in my logic.
 
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  • #2
Hey Fp! :smile:

Only saw this thread just now.

Weren't you giving lessons in mechanics nowadays?

Anyway, you are assuming that all cables give the same tensional force.
They don't.
It's not all the same cable.
There are 3 cables, each with its own tensional force.
 
  • #3
I am giving lessons in mechanics, but with a very focused set of materials that I'm well-practiced with. In our course we never gone through several cables connected to system of pulleys. In fact, we hardly gave pulleys much time. At most we had one cable if I recall correctly.

Where do you see 3 cables?
 
  • #5
And I just solved 2 more such exercises correctly. Don't make me prove myself :) Just needed to understand one "Graphical" principle!
 
  • #6
Very good! You are :cool:

ehild
 
  • #7
Good!

If you feel up to the challenge... what would happen if there was indeed 1 cable and holes in the pulleys?
 
  • #8
Well then P would equal 1/6 of the weight. :)
 
  • #9
Femme_physics said:
Well then P would equal 1/6 of the weight. :)

Suppose P would equal 1/6 of the weight, what would be the resultant force on the leftmost pulley?
 
  • #10
I like Serena said:
Suppose P would equal 1/6 of the weight, what would be the resultant force on the leftmost pulley?

2/6 of the weight
 
  • #11
Femme_physics said:
2/6 of the weight

That would mean that ƩFy is not zero... what would that mean?
 
  • #12
I like Serena said:
That would mean that ƩFy is not zero... what would that mean?

Of course ƩFy is zero. The resultant force at Y of the rightest pulley is 2/6... Of the middle pulley is also 2/6...and the right 2/6... these are the up vectors in the Y direction...the down vector is just the weight...so of course it ends up being ƩFy=0
 
  • #13
Femme_physics said:
Of course ƩFy is zero. The resultant force at Y of the rightest pulley is 2/6... Of the middle pulley is also 2/6...and the right 2/6... these are the up vectors in the Y direction...the down vector is just the weight...so of course it ends up being ƩFy=0

Which up vectors do you mean?

I see only 2 downward tensional forces on each pulley and the pulleys are not attached to anything else (since we assumed that the cables were not attached to the pulleys).
 
  • #15
Femme_physics said:
Each pulley has two!

Ah, I meant that I didn't see any up vectors on for instance pulley A.
 
  • #16
Wait a second...That was a trick question! If there are no pins attaching the pulleys to the rope they're going to slide down!
 
  • #17
Femme_physics said:
Wait a second...That was a trick question! If there are no pins attaching the pulleys to the rope they're going to slide down!

Yep. The whole thing would collapse! :eek:
 
  • #18
:) Thanks for the stimulation!
 
  • #19
Femme_physics said:
:) Thanks for the stimulation!

I'm just thinking of those poor cardboard men that were standing below that weight! :bugeye: :wink:
 
  • #20
LOL! Remeber them eh? :)
 

1. What is statics mechanics?

Statics mechanics is a branch of physics that studies the behavior of stationary objects under the influence of forces. It involves analyzing the forces acting on a stationary object to determine its equilibrium and predicting its motion.

2. How is statics mechanics different from dynamics mechanics?

While statics mechanics deals with stationary objects, dynamics mechanics deals with the motion of objects under the influence of forces. In statics mechanics, the object is at rest or in constant motion, whereas in dynamics mechanics, the object is accelerating or decelerating.

3. What are some common applications of statics mechanics?

Statics mechanics is used in various fields such as engineering, architecture, and construction to design and analyze structures and machines. It is also used in the development of tools and equipment, as well as in the fields of geology and astronomy to understand the forces acting on celestial bodies.

4. How do you determine the equilibrium of an object in statics mechanics?

To determine the equilibrium of an object in statics mechanics, the sum of all the forces acting on the object must be equal to zero, and the sum of all the torques acting on the object must also be equal to zero. This means that the object is either at rest or moving at a constant velocity.

5. What is the importance of statics mechanics in real-world scenarios?

Statics mechanics is crucial in understanding and predicting the behavior of structures and machines in real-world scenarios. By applying the principles of statics mechanics, engineers and architects can design stable and safe structures, while also optimizing the use of materials and resources.

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