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DocAl said:Only two forces act on the rod: Gravity and the GRF. Since gravity acts vertically, only the horizontal component of the GRF creates the horizontal acceleration of the center of mass. That's why we're trying to find the acceleration of the center of mass.

I understand that net external forces are needed to produce a change in momentum (Newton's second). I run into difficulty with cause and effect. To me it seems that the falling rod's weight causes GRF so to count the effect of gravity and the effect of GRF seems like counting the same force twice.

To help me resolve this confusion (I know I am wrong), can you explain in words how the GRF comes into being?

DocAl said:This won't work because you ignored one of the forces acting on the rod--the GRF also has a radial component. To use Newton's law you'd need the net radial force, not just the radial component of the weight.

OK, so we have the acceleration from weight, a = g cosθ and the force from GRF. The GRF is what I am trying to calculate, so I cannot find the net radial force like this.

DocAl said:What I wanted you to do was calculate the radial acceleration kinematically. Are you familiar with the standard formulas for centripetal acceleration?

I've just had a look and I think F = m|v|^2 / R is the kinematic equation for centripetal force.

In the example, that gives F = |v|^2

Now we need to resolve v, so we can use the standard equation of motion v = u + at, but that would introduce the time variable into our equations which does not seem useful - I'm guessing there is a better approach?