# Keeping This Unit Coherent through Gravitation

The unit I initially dreaded keeping coherent was a unit that included:

- Uniform Circular Motion
- Universal Law of Gravitation
- Angular Kinematics
- Torque and Angular Dynamics
- Static Equilibrium

So far, how we have managed to keep it coherent is through gravitation. After circular motion, we went into the Universal Law of Gravitation. As I’ve blogged about, we spent a fair amount of time talking about the Cavendish experiment. Armed with the Universal Law of Gravitation, students worked a problem in class to calculate the mass of the sun. It’s a pretty cool calculation to work, because all you need to know is the length of the year and the distance to the sun. We give students the mass of the earth (so groups can approach it through piece-wise calculations), although many students realize they don’t need it.

Then, angular kinematics, torque, and Newtons’ 2nd Law for Rotation culminated in us returning to the cavendish experiment Students had a challenge problem of calculating how muchÂ angular acceleration the rod would experience. This forced them to draw upon their knowledge of universal gravitation and angular dynamics. We then used angular kinematics, estimating how long it should take the rod to turn (~.06 radian), using the not quite correct assumption that the angular acceleration is constant. We get a good estimate, of 10-11 minutes.

Although I’ve experienced it as a coherent, I know enough to know that doesn’t necessarily mean the students have.