Qualitative Cavendish Experiment: Prepping for PiM Lessons on Gravitation

Next week, we’ll be learning about circular dynamics and the universal law of gravitation. I plan on going through the Preconceptions in Mechanics lesson, very similar to the lesson as described by Frank. In the lesson, students are asked about to think about various possible causes of gravity (earth’s spin, earth’s magnetic field, earth’s mass, etc). They observe demos and discuss in a sequence. One might typically end the lesson by watching a video or reading about Cavendish’s famous experiment.

So anyway, over fall break, I got the itch to actually set up the cavendish experiment for myself–qualitatively that is. I originally set it up on the ceiling cross bars in the physics room, but these are just some flimsy aluminum cross bars that hold up paneling. I ended up moving my setup to a door frame to keep vibrations from the building from interfering. I’ve run the observation experiment dozens of times now, and was even able to show it live to a friendly colleague from Speech and Pathology. I’m pretty convinced that what I’m seeing is the gravitational effect, because I can get it with 100% reliability in both directions. There are few tricks to getting it working well, mostly just giving it time to reach equilibrium, and being careful when moving the bricks. But it’s really not terribly difficult. This could easily be a good project for students.

Here is what my setup includes and looks like:

• 1 meter stick
• 2-200 gmasses
• 2 lead bricks
• fishing line

Here is a time-lapse video of a setup with a big deflection. The actual video is about 10 minutes long. I used i-phone time-lapse to make this. [Edit: This video, likely has acceleration from torsion in the string. Andy helped point out that this much deflection would likely take more time than reasonable]

I also have a time lapse video with a smaller deflection zoomed in on just one side: [Edit: This video seems more reasonable]

I plan on at least setting up the experiment for students and showing the video. I may venture into actually showing the demo in class, but I’m worried it will be hard to do. I may want to make a better video that’s not so nauseating. I’ll also let you know if I find time to approach the experiment quantitatively to estimate G.

Let me know if you have any questions about the setup.

Edit:  In general, what I’ve learned is it can take a long time for the system to settle into equilibrium before you bring the lead brick nearby. The next day when I went back to do experiments, people in the upstairs gym were playing basketball, and the system never reached equilibrium. The pendulum would twist 5 degrees or so back and forth over — over long periods of time. Moving to a building that is really isolated from vibrations is important.