I haven’t been teaching physics very long, but in my few years I’ve begun to notice that there are a few concepts in physics over which students tend to fold their arms across their chests. When you tell a student about Isaac Newton’s third law of motion, they nod and smile—until you start talking about gravity. They grow suspicious when you tell them that they exert an attractive force on Earth. “Sure, the Earth pulls me down, but come on. Am I really supposed to believe that I pull the Earth up with the same kind of force?” Then I tell them that all objects with mass attract all other objects with mass, and my credibility is gone.

The idea to set up a mock-Cavendish experiment came to me when I was teaching Newton’s law of universal gravitation to my AP Physics C: Mechanics students. You can read a bit about the history of Henry Cavendish and his experiment on Inside Science.

I showed them a video on YouTube that was produced by a man named Brady Haran, and that got the whole class thinking: “Mr. Rennekamp, couldn’t we do this experiment? If we’re really careful, we could duplicate Henry Cavendish’s measurement for the gravitational constant.” I was intrigued and gave them the assignment to research the feasibility of recreating the Cavendish experiment.

The next day we sat down to discuss our findings and concluded that we would have a big problem with precision on our current project budget of, well, zero dollars. Too many variables would lie beyond our control. So we gave up on measuring gravity reliably on a small scale, but we thought we could at least demonstrate that the force still works on a small scale. The change of goal would help the more critical students to see with their own eyes that the force of gravity works on all objects with mass.

Many of the students had seen video footage of the experiment sped up to five or ten times normal speed. They found the effect to be very convincing, so as a class, we decided to film our experiment and then speed up the footage to make the acceleration noticeable.

After a few design iterations, we settled on the following apparatus. We found a room in the school that is quiet and unused. We hung a torsion balance from the ceiling. The balance consisted of a strand of fishing line fixed to the drop ceiling by looping the line over the ceiling grid. The line ran down to a horizontal rod. For this, we used two meter sticks that we latched together using tape or rubber bands. Two meter sticks were needed to maintain symmetry with the fishing-line coming down between them and to prevent the torsion balance from hanging at a weird angle. The figure below details the place where the fishing line meets the meter sticks.

To help balance their torsion bar, Rennekamp and his students used two meter sticks and ran the fishing line between them, as shown here. CREDIT: Anthony Rennekamp

We then put concentrations of mass on the ends of the meter sticks. We had a few 1-kg cylinders with hooks on top, which we simply hung on each end. After a bit of playing with the placement of the masses, we had a torsion balance that remained perfectly horizontal. At this point we had to leave the room to give the torsion balance a chance to find equilibrium.

After a while we came back with large masses. We decided to borrow 50-lb-free-weights from the strength and conditioning room. We began filming and then quickly placed the weights on the floor offset in a counterclockwise circle from the ends of the torsion balance. Then, we quickly left the room for 30 minutes. When we came back, we had footage to speed up and view. We then repeated the process in the clockwise direction.

The Bishop O'Connell Cavendish experiment. CREDIT: Anthony Rennekamp

The results certainly convinced us. Sped up by a factor of five, the footage showed that the masses on our torsion balance tended to accelerate toward the free-weights, regardless of orientation—be it clockwise or counterclockwise. I asked my students to write about what they learned throughout the production of this experiment. Many students said that seeing the footage gave them a concrete experience on which they could build understanding of the concept of gravity. Some of them noted that they wished we could have calibrated the torsion balance so that we could measure the force of gravity and compare their measurement to Newton’s Law of Gravitation.

Every student commented on how much they enjoyed the design and execution of the experiment. It gave them an opportunity to design something that would be used to teach others. As they rendered their design, they had to constantly keep this question in mind: “If I wasn’t convinced that all objects have gravitational pull, would I be converted by this footage?” That priority forced them to rigorously scrutinize their methods and adjust their design until they had a finished product that was convincing to others.

For a first attempt, I am happy with the quality of experiment my students designed and intend to repeat the Cavendish experiment in my class in future years. A harsh critic looking at our footage could point to a few factors that discredit our claim that all mass is mutually attractive. For example, the students were not careful enough to ensure that the torsion balance’s equilibrium was consistent between experiments.

In the future, I will try to give them more tools up front—such as the meter sticks and the fishing line—and place myself in the role of spectator and coach. Doing so would allow them to concentrate more on the particulars of the design and its perception from critics rather than the process of procuring the experimental material themselves.

On a more logistical note, I was surprised by how quickly the apparatus the students created manifested acceleration. I had expected the masses on the torsion balance to take close to an hour to finally hit the free-weights, but in the sped-up footage, they began to visibly move toward the 50-lb masses immediately. In fact, the masses covered the approximately six-inch displacement in less than ten minutes. The relatively quick results will enable future classes to run several iterations within the span of one 45-minute class period. They can use the next class period to troubleshoot and edit footage.

In future years, I will introduce the experiment by showing the students the Cavendish video on YouTube, followed by the footage the classes took in previous year. We’ll see if next year’s students come up with a video that is more convincing.

I am now teaching AP Physics C: Electricity and Magnetism and thinking about famous experiments that my class could re-enact on a tight budget. We could explore the connection between electricity and magnetism using apparatus’ designed by Hans Christian Ørsted or Michael Faraday.

Such recreations help to give students historical context while immersing them in the concepts we are studying. I am very pleased to be able to do these experiments with my students beyond the normal inquiry labs we do, and delighted by their enthusiastic response.

Anthony Rennekamp is a physics teacher at Bishop O'Connell High School in Arlington, Virginia.