# Homework Help: Statics: a hinge held up by a string with a mass

1. Apr 12, 2012

### bfusco

1. The problem statement, all variables and given/known data
A uniform beam of mass M and length L is mounted on a hinge at a wall as shown in the figure. It is held in a horizontal position by a wire making an angle (theta) as shown. A mass m is placed on the beam a distance from the wall, and this distance can be varied.

http://session.masteringphysics.com/problemAsset/1058226/6/GIANCOLI.ch12.p84.jpg

a)Determine, as a function of x, the tension in the wire.
Express your answer in terms of the variables m, M, L, x, θ, and appropriate constants.

b)Determine, as a function of x, the horizontal component of the force exerted by the hinge on the beam. Assume that the positive x and y axes are directed to the right and upward, respectively.

c)Determine, as a function of x, the vertical component of the force exerted by the hinge on the beam.

3. The attempt at a solution
a)first i realized that the force from the hinge and force due to tension had components.

choosing the hinge for the torque axis of rotation, i used the equation ƩFx=0, Fhx-Tx=0, Fhx=Tx. Then i used ƩFy=0, Fhy+Ty-mg-Mg=0 (here i didn't know what to solve for) and finally i used the equation Ʃτ=0, LT-xmg-(L/2)Mg=0, T=(xmg+(L/2)Mg)/L and it is at this point i dont see how i have yet to solve for anything useful to simplify any of the other equations.

2. Apr 12, 2012

### tal444

I'm not sure why you would need to solve for anything. Isn't the question simply asking for an equation involving those variables?

3. Apr 13, 2012

### azizlwl

Torque=F.r.Sinθ

4. Apr 13, 2012

### bfusco

yes, but im focusing on solving for the force due to tension (FT). so i somehow need to work the equations to solve for FT while taking the Fh (force from the hinge) out of the equation. With the Fh in the equation there are 2 unknown variables and that wouldn't be a function of x.

5. Apr 13, 2012

### tal444

I'm confused, ignore the hinge for the moment if it's your axis. Your equation should be something like the (force of center of gravity)(length acting on) + (force of mass)(length acting on) = (tension on cable)(length acting on). Rearranging that equation, I see no Fh that you speak of.
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