Diagram is attached.(adsbygoogle = window.adsbygoogle || []).push({});

P = forces

M = free-moving moment

C = looks like a pin connection

L = distance between A and C, and C and B

With the system in equilibrium.

Question: Write the moment equations about point A, B and C and prove that they are all the same.

so basically I end up with:

(Counter-clockwise moment is positive.)

Summation of moments about point A: P(sin(90-theta))(2L) - M - Cx(sin(90-theta))(L) + Cy(sin(theta))(L) = 0

point B: P(sin(theta))(2L) + Cx(sin(90-theta))(L) - Cy(sin(theta))(L) - M = 0

point C: P(sin(90-theta))(L) - M + P(sin(theta))(L) = 0

which means that

P(cos(theta))(2L) - Cx(cos(theta))(L) + Cy(sin(theta))(L) = P(sin(theta))(2L) + Cx(cos(theta))(L) - Cy(sin(theta))(L) = P(cos(theta))(L) + P(sin(theta))(L) = M.

I don't see how these can be equal. Any mistakes seen here in the approach to solving this problem? Maybe pin supports Cx, Cy don't create moments?

Thanks for the help!

**Physics Forums | Science Articles, Homework Help, Discussion**

The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

# Homework Help: Moment problem (statics)

**Physics Forums | Science Articles, Homework Help, Discussion**