Equilibrium of a Rigid Body Under Coplanar Forces

[Jordan_]

I have a pretty tricky question here and I can't seem to figure it out. I just maybe need a slight hint?

A truss is made by hinging two uniform, 150N rafters. They rest on an essentially frictionless floor and are held together by a tie rope. A 500N load is held at their apex. Find the tension in the tie rope. ANS: 280N

Both rafters are 3m long and the tie rope is tied around them 0.5m from the bottoms.

I've been playing with this for quite a while now so any little hints would be appreciated. Maybe something that could put me in the right direction. Like for instance I'm having trouble knowing where to draw the FBD from. The hinge at the top? Or the bottom?

Thanks

 

[Pyrrhus]

If both contact points of the beams are aligned (string is horizontal) then you can take moment by one of the contact points and find the normal force, then you can take moment by the apex and find the tension.

 

[Jordan_]

Okay thanks man! I used what you said and eventually figured it out. Interesting how it looks so hard but once you know how to do it - it seems almost like common sense :shy: .

I have one more though. I have done most of the work, but the answer I'm getting isn't the right one. Let's see what you think.

For the rotating object below determine the center of rotation, as well as the tension in the masless rods, if w = 1.0 rad/s.

(Picture of a triangle, each point being a ball, connected with rods to each other.

An equilateral triangle. All masses are 1.0kg.

ANS: (Rcm = 0.5, 0.29) and (T = 1/3 N)

To get the center of mass I did:

Rcm = [1(0, 0) + 1(0.5, 0.87) + 1(1, 0)]/3

Rcm = (0.5, 0.29)

Once I had that, I went for the radius from my starting point (0,0) to it's center of mass. Using the pyththeorem c^2 = 0.5^2 + 0.29^2 I got c = 0.58.

Using the formula:

Fnet = (m)(w^2)(R)
T = (1)(1^2)(0.58)
T = 0.58 N ?

Not the correct answer it seems. Where have I gone wrong?

 

[SS2006]

i can't see anything you may have done wrong, anyone else see diffrently