Solving PSE6 6.P.011: Find Tension in Strings

[ramin86]

Everyone I know is having problems with this question:

3. [PSE6 6.P.011.] A 3.85 kg object is attached to a vertical rod by two strings as in Figure P6.11. The object rotates in a horizontal circle at constant speed 7.30 m/s.

Figure P6.11

(a) Find the tension in the upper string.
N
(b) Find the tension in the lower string.
N

http://www.webassign.net/pse/p6-11.gif for the figure

I've tried using various force equations, only to be wrong. Does anybody know how to do this problem??

 

[ramin86]

Anybody know how to do this problem? Tried a lot of things, but I'm down to my last chance to answer the question correctly

 

[wisky40]

it seems you are desparate, just show me your "force equation and I help you out right know

 

[ramin86]

Well I tried some forms of the centripetal acceleration formula for the upper tension, and I also tried 3.85*9.88(cos of the angle) + 1/2 the horizontal force, but they haven't worked out.

 

[wisky40]

Only because you try to do something, I'm going to help.
Use these forces ==>
T"sin(alpha)-T'sin(alpha)-mg=0 and T"cos(alpha)+T'cos(alpha)=mv^2/r
where T" is the upper tension and T' is the lower tension. Also, if you want to find
cos(alpha) and sin(alpha) use your sketch