# Equilibrium value of an angle

1. Oct 15, 2008

### davesface

A uniform thin rod of length L and mass M is pivoted at one end. The pivot is attached to the top of a car accelerating at rate A.

a.) What is the equilibrium value of the angle $$\Theta$$ between the rod and the top of the car?

Honestly, I'm not entirely sure what the question even wants me to find. Playing the problem out in my mind, it seems like the angle would just continue to increase at a constant rate as the car accelerates with a constant rate A, but clearly this is not the case.

b.) Suppose that the rod is displaced a small angle $$\Phi$$ from equilibrium. What is its motion for small $$\Phi$$?

I would assume that the motion would be towards equilibrium, although the vertical component of motion for the rod itself should still be the same, as the approximation cos$$\Phi$$=1 should hold for small values of $$\Phi$$.

2. Oct 16, 2008

### tiny-tim

Hi davesface!

Let's do a.) first …
Hint: in equilibrium, the acceleration of the end of the rod will be the same as the acceleration of the car.

So use good ol' Newton's second law

3. Oct 17, 2008

### davesface

OK, part A was a lot easier than I thought, and all I did was set the fictional force backwards equal to the force of weight and solve for theta.

Turns out that part B would've required us to solve a differential equation, which no one knows how to do. Thanks for the response, though.