Using How-focused Examples from Science to Support Students and Teachers ~Henry Cavendish

Using How-focused Examples from Science to Support Students and Teachers ~ Henry Cavendish Measures the Strength of Gravity

     In 1798, a scientist effectively “weighed” the Earth — without leaving his laboratory. The English scientist Henry Cavendish designed an incredibly sensitive experiment. Inside a quiet wooden shed, he hung a horizontal rod from a very thin wire. Two small lead spheres were attached to the ends of the rod. Nearby, he placed two much larger lead balls. Because of gravity, the large spheres slightly pulled the smaller ones. The force was extremely tiny — so small that the rod twisted by only a minute fraction of a degree. Yet that tiny twist held a big secret. By carefully measuring this small movement, Cavendish determined the strength of the gravitational attraction between objects. From this, scientists could calculate the mass of the entire Earth. His estimate was remarkably close. Cavendish calculated Earth’s mass to be about 6 × 10²⁴ kilograms, while modern measurements give 5.97 × 10²⁴ kilograms. Sometimes the biggest discoveries come from noticing and measuring the smallest forces.

     What Cavendish performed was one of the most elegant examples of inference and Engineering Design Processes in scientific history.  Cavendish didn’t literally “weigh” the Earth; he solved for a missing variable in a universal equation.

Here is how measuring those tiny lead spheres revealed the mass of the entire planet:

The Inverse Relationship: Newton’s Law of Universal Gravitation states that the force of gravity depends on two things: the masses of the objects and the distance between them.

The Earth’s Pull:  Cavendish already knew how hard the Earth pulled on an object – that was just an object’s weight.

The Missing Link: What Cavendish did not know was the “constant”- exactly how much gravitational force (G) a specific amount of mass exerts.

Scaling the Experiment: By using small lead spheres in a controlled environment, Cavendish created a “miniature version” of gravity. Because he knew the exact mass of the lead balls and the distance between them, he could measure the tiny force they exerted on each other (the “twist”). This allowed Cavendish to calculate the Gravitational Constant (G).

 The “Aha!” Moment: Once Cavendish knew the value of G, the Earth became just another object in the equation. Cavendish knew the radius of the Earth (distance). He knew the force Earth exerted on objects at its surface (weight). By plugging the newly discovered G into the formula, the Mass of the Earth was the only unknown left to solve. 

 Why Lead? Cavendish used lead because it is incredibly dense. To measure such a weak force, he needed as much mass as possible packed into a small volume to maximize the gravitational “tug” while keeping the equipment small enough to fit in his shed. Essentially, by measuring how two hand-sized balls moved, Cavendish found the “density” of the universe’s glue, which allowed him to “weigh” anything from a mountain to a planet.

Here is a link to a short video depicting how the Cavendish experiment worked~ https://x.com/zackdfilms1/status/2031023230492786907 

How-Focused Support: Using Science to Support Questions

How-focused Supporting Question (STEM): How can the EDP process and the Scientific Method used by Cavendish serve as another example that you might share with your Mentee?

or 

How-focused Supporting Question (Other Subjects): How might the experiences of Cavendish serve as a helpful example that you might share with your Peer Teachers and/or your Students for either subject-matter or Social Emotional Learning (SEL) support?

                                                               References

Teague, H. (2026, March 9). Using how-focused examples from science to support students and teachers ~ Henry Cavendish. Edublogs. https://4oops.edublogs.org/2026/03/12/howfocusedsteam_cavendish/
Adapted from an X post. 

More Weekend Ed. Quotes

#GCUTEC544 #GCUTEC595 #GCUTEC516
#CUNE604, #CUNE605 #CUNE603