Nova Physics: Chapter 7-Example 3

[InquisitiveDoc]

Now, I'm not the best physics student so this question definitely had me stumped. For those of you who have the book and have made sense of that example, i'd appreciate the help. For those of you who don't have the question, here goes. (sorry, i can't provide the drawing) It's a part of the Torques chapter.

A pole of length L is connected to a hinge at point A on a vertical wall, making an angle a with the wall. A horizontal string connects to the wall at point B and the end of the pole at point C. A box of candy of mass m hangs from a string at the end of the pole.
1- what is the tension in the horizontal string?
2- what is the magnitude of the force of the wall on the pole at point
A?

*now the book used point A as the pivot and reached this answer:
T=mg(sin(a)/cos(a))
AND
F= -(mg)/(cos(a)): it goes on to explain that the F is negative because the direction chosen for force of the wall on the pole was incorrect (it was compressional and not tensile).

Can any of you that are good at physics explain this to me?? Thanks a bunch!!

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[zayka]

A scan of the drawing would be really helpful. Otherwise, I can't help you - not really sure what it means by point C, etc.

 

[dedicate]

Sounds like a good question, but does it include a picture?

 

[physics junkie]

Having a tablet can be helpful. Is this what the diagram is supposed to look like?

 

[physics junkie]

Just realized the one I posted doesn't fit the criteria. How about:

 

[InquisitiveDoc]

Just realized the one I posted doesn't fit the criteria. How about:

THIS IS EXACTLY HOW THE DIAGRAM LOOKS!!! THANKS SO MUCH BECAUSE I WAS BEGINNING TO THINK THAT I WOULD GET NO HELP BECAUSE I DONT HAVE A SCANNER!!! so now that we have a diagram, can someone explain to me the problem.

i understand the ideas of torque balance. what i dont understand is the use of different points such as point B or point A to eliminate certain forces and concentrate on others.

am i alone in this state of confusion??

 

[Agent47]

Hmm...I'm using Kaplan and TPR, I have always had problem where Pole is Horizontal to the Wall so we can use that as Pivot point and make the force Verticle and horizontal to that pivot point Zero...since the Cosine would be 90 degree at that point!!!!

In your problem they are doing kind of opposite...I guess the only thing you have to do is find the T on the horizontal!!! Using the unknown angle that pivot point makes with the Wall will be same that string makes with bar. If you kind of draw a line between the Point B and Candy box ...You will see that T= W Tan a.

This is what I saw!!! No 100% sure though!!!

 

[Vihsadas]

These kinds of posts belong in the Study Question Q&A forum.

 

[physics junkie]

THIS IS EXACTLY HOW THE DIAGRAM LOOKS!!! THANKS SO MUCH BECAUSE I WAS BEGINNING TO THINK THAT I WOULD GET NO HELP BECAUSE I DONT HAVE A SCANNER!!! so now that we have a diagram, can someone explain to me the problem.

i understand the ideas of torque balance. what i dont understand is the use of different points such as point B or point A to eliminate certain forces and concentrate on others.

am i alone in this state of confusion??

You're welcome. I'll help you out by wednesday-ish if no one else does. I'm pretty busy right now.