Need help with Statics HW...

[JohnCU]

figure 2.60

Knowing that portions AC and BC of cable ACB must be equal, determine the shortest length of cable which can be used to support the load shown if the tension in the cable is not to exceed 725 N.

The part that is throwing me off is the tension in the cable. Are they talking about the tension in AC or BC or what? or is it 362.5 in each one? I think this has something to do with isosceles triangles, but I can't figure it out.

 

[Random Variable]

I have that textbook--Vector Mechanics for Engineers by Ferdinand P. Beer and E. Russell Johnston.

 

[Triumph]

Listen, you have three equations in all of statics. One of which you don't even need here. Sum the forces in X and Y. If you don't know how to break a force down to its components, then well....

 

[JohnCU]

yeah I've tried that, but the two forces in the x cancel each other out...?

 

[Heisenberg]

Originally posted by: JohnCU
yeah I've tried that, but the two forces in the x cancel each other out...?

That's the general idea, yes.

 

[JohnCU]

right, but then you have a 1000 N force going down, so how can you only have a 725N force going upward?

 

[Triumph]

yes, they do cancel. and the two forces in the Y direction = 1000. and another hint is that statics is almost all right triangles, not isosceles

 

[ArmenK]

Originally posted by: JohnCU
yeah I've tried that, but the two forces in the x cancel each other out...?

IT'S CALLED STATICS!

 

[JohnCU]

am I suppose to add AC and BC together, then make it only 725 N?

 

[Yossarian]

Originally posted by: JohnCU
The part that is throwing me off is the tension in the cable. Are they talking about the tension in AC or BC or what? or is it 362.5 in each one? I think this has something to do with isosceles triangles, but I can't figure it out.

a cable only has one tension, at least for theoretical problems like this.

 

[Triumph]

no.

 

[Triumph]

hint: write an equation for the angle made by AC and the vertical.

 

[JohnCU]

i'm an EE, i shouldn't have to do this kinda crap.

 

[JohnCU]

umm...sin(AC) = 1.5/A ?

 

[Gibson486]

Originally posted by: JohnCU
i'm an EE, i shouldn't have to do this kinda crap.

go find my opamp question then

 

[Triumph]

Originally posted by: JohnCU
umm...sin(AC) = 1.5/A ?

mmmm, no. you can only take the sin of an angle, not a side.

 

[Triumph]

and you need to learn your trig before you get to imaginary numbers, which are needed for all of your AC circuit analysis.

 

[JohnCU]

1.5/A = sinx, opposite over hypotenuse, correct?

I know AC circuit analysis...

 

[Triumph]

so now you have two unknowns in that equation. where can you get another equation so that # unknowns = # equations.

i always think of force vectors as ratios. it isn't directly correlated to percents, but you can think like, so much of the force goes in X, and the rest goes in Y. the geometry is what gets you those ratios.

 

[JohnCU]

yeah i know, Fcosz = Fx, Fsinz = Fy, etc etc etc...but I can't find the second equation.:disgust:

 

[Triumph]

Sum (Fy) = 1000.

 

[JohnCU]

yeah Fa(1.5/A) + Fb(1.5/B) = 1000 N, I've had that equation for an hour now but don't understand, since the tension can only be 725 N in the cable, how is it gonna support 1000 N?

 

[JohnCU]

nevermind still lost.

 

[helpme]

Originally posted by: JohnCU
i'm an EE, i shouldn't have to do this kinda crap.

Haha, I say the same thing. Statics + Dynamics this quarter since I was too lazy to take 3 years ago.

 

[Triumph]

Two hints before I go to bed.

The resultant of Fx and Fy is equal to 725.
Fy = 500.