# Homework Help: Simple dynamics equation

1. Feb 8, 2013

### zoom1

Didn't get this equation which is written for the pendulum part of the cart.
Must be easy but couldn't get it.

2. Feb 8, 2013

### AlephZero

It is just "force = mass x acceleration" in the x direction, for the center of mass of the rod. (The length of rod isn't shown on your diagram, but if the equation is correct it is 2l, not l).

The $\ddot\theta$ term is from the tangential acceleration of the rod.

The $\dot\theta^2$ term is the from the radial (centripetal) accleration.

3. Feb 8, 2013

### zoom1

It's a little clearer now, but didn't understand the centripetal force component.

Isn't it

4. Feb 8, 2013

### AlephZero

Yes.

You also know $v = \omega r$ and $\omega = \dot\theta$

The centripetal force is radial (along the length of the rod), but the equation is for the component of the force in the X direction.