A nonuniform, horizontal bar of mass m is supported by two massless wires against gravity. The left wire makes an angle phi_1 with the horizontal, and the right wire makes an angle phi_2. The bar has length L. (Intro 1 figure)

<img src=”http://session.masteringphysics.com/problemAsset/1…

Find the position of the center of mass of the bar, x, measured from the bar’s left end.

Express the center of mass in terms of L, phi_1, and phi_2.

I typed in x = L / ( (tan(φ₂)/tan(φ₁) + 1), but its telling me that its incorrect, what do I do?

### 3 Answers

Your answer, I think, is incorrect.

-1st, you have to write the equation for Torques about left end of bar:

L*T_2*sin(phi_2) – mgx = 0

– Equation for the x-components of the forces:

T_2*cos(phi_2) – T_1*cos(phi_1) = 0

==> T-1/T-2 = cos(phi-2)/cos(phi-1)

– Equation for the y-components of the forces:

T-1*sin(phi-1) + T-2*sin(phi-2) – mg = 0

==> mg = T-1*sin(phi-1) + T-2*sin(phi-2)

– Find x from the 1st equation. To eliminate mg from the 1st equation, replace mg by T-1*sin(phi-1) + T-2*sin(phi-2)

Thus,

x = T-2*sin(phi-2)*L / (T-1*sin(phi-1) + T-2*sin(phi-2))

Divide top and bottom by T-2*sin(phi-2), you have:

x = L / (T-1*sin(phi-1) / T-2*sin(phi-2) + 1)

From 2nd equation, you have T-1/T-2 = cos(phi-2)/cos(phi-1), replace this to the above x equation to eliminate T-1/T-2

The answer is x = L tan(phi_2)/tan(phi_2)+tan(phi_1)

don’t forget the parenthesis – causes an error…lol took 19 tries

x=L/ (tanφ₁*cotφ₂)+1