Static equilibrium/tension force problem!

  • Thread starter psneath
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  • #1
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Homework Statement



The attached system is in static equilibrium, and the string in the middle is exactly horizontal.
Find:
a)tension T1
b)tension T2
c)tension T3
d)angle theta

Homework Equations



I know I need to create equations that equal zero to analyze the x and y components of force as well as torque

The Attempt at a Solution



So
Fx=T3x-T1x=0 (I think the tension in T2 is zero)
Fy=T1y + T3y-m3g-m2g=0
For the torque equation I'm using the axis of rotation as the point where T1 meets m3

torque = T2R + T3yR - m2gR

When I try to solve any of these I come across way too many unknowns to solve...can someone see if I've assessed for forces properly and show me where to go from here?

Thanks!

Homework Statement





Homework Equations





The Attempt at a Solution

 

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Answers and Replies

  • #2
K29
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you have 4 unknowns and 3 equations which means you should be able to solve for each variable fairly easily. ( I say 4 unknowns because T3x cand T3y can both be expressed in terms of T3, with some triangular thinking.. Not sure if your torque equation is right or wrong(haven't done that stuff in a while)
 
  • #3
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Can I assume that T1y=m3g and T3y=m2g?
 
  • #4
collinsmark
Homework Helper
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Can I assume that T1y=m3g and T3y=m2g?
Yes, you can and you should. :biggrin: [I assume you mean T1y = (3.0 kg) x g, and T3y = (2.0 kg) x g]

And it's important to understand why you can. So I encourage you to think about it for a couple of minutes.

Consider a tiny point on the string, right at the intersection of the T1 string, the horizontal T2 string, and the vertical string holding up the 3.0 kg mass. According to this small point of string, there are only 3 forces acting on it. So you can isolate this particular point to be a function of only these 3 forces. How many forces are acting on that particular point in the y-direction? Since nothing is accelerating, what does it tell you about those forces that exist in the y-direction (as seen from that particular point)?

[Edit: Btw, The above applies because we are dealing with light strings. If we were dealing with heavy rods, or rigid rods that clamped onto each other and held firm, things might be a little different.]
 
Last edited:

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