The Cavendish Experiment is the 1798 torsion-balance experiment that measured the tiny gravitational pull between lead masses. In Principles of Physics I, it shows how Newton’s law can be tested and used to find G and Earth’s density.
The Cavendish Experiment in Principles of Physics I is the classic torsion-balance setup Henry Cavendish used to measure the very small gravitational attraction between known masses. Instead of trying to “see” gravity directly, the experiment turns that pull into a measurable twist in a wire. That twist gives you the force between the masses.
The basic setup uses a horizontal rod with small lead spheres on the ends, suspended by a thin fiber or wire. Two larger lead spheres are placed nearby. Their gravity pulls on the smaller spheres, the rod rotates a tiny amount, and the fiber twists back like a spring. The balance point between the gravitational torque and the torsion in the wire is what makes the measurement possible.
That is the clever part of the experiment: gravity between lab-scale objects is extremely weak, so you need a device that can detect a force so small it would be hard to measure any other way. The torsion balance converts a tiny force into an angular deflection, which is much easier to measure carefully. Once you know the geometry of the setup and the twisting constant of the wire, you can work backward to the gravitational force.
In the course context, this experiment connects directly to Newton’s Law of Universal Gravitation. The measured force depends on the masses of the spheres and the distance between them, matching the inverse-square relationship. Cavendish did not write down a new law of gravity here, but he gave physics one of its first high-precision measurements of the gravitational constant, G.
It also explains why the experiment is tied to Earth’s density. If you know G and you can compare the pull of Earth to the pull between the spheres, you can infer Earth’s mass and then its average density. So the experiment is both a force measurement and a way to connect a lab-scale result to a planet-scale quantity.
The Cavendish Experiment matters in Principles of Physics I because it shows how a law can move from a formula on paper to a measured quantity in the lab. Newton’s law tells you that masses attract each other, but Cavendish showed how to measure that attraction when the force is tiny.
That makes it a strong example of experimental reasoning in mechanics. You are not just plugging numbers into F = Gm1m2/r2. You are seeing how a physical apparatus, a torsion balance, turns a nearly invisible gravitational force into a measurable twist and then into data you can calculate from.
It also helps separate three ideas that are easy to blur together: the gravitational force between objects, the gravitational constant G, and Earth’s density. The experiment does not directly “weigh Earth” with a giant scale. Instead, it measures a small interaction in the lab and uses that result to infer a much larger property.
In class, this kind of problem often shows up as a bridge between theory and measurement. If you can explain why the balance turns, identify which quantities are known or unknown, and connect the data to Newton’s law, you are showing that you understand both the physics and the method behind it.
Keep studying Principles of Physics I Unit 12
Visual cheatsheet
view galleryTorsion Balance
The Cavendish Experiment depends on the torsion balance, which is the device that turns a tiny sideways gravitational pull into a measurable angular twist. If you understand the balance, the experiment stops looking like a historical demo and starts looking like a force measurement tool. The fiber’s resistance to twisting is what lets the setup settle at a measurable angle.
Gravitational Constant
Cavendish’s result is tied to G because the measured force between the masses can be plugged into Newton’s law to solve for the constant. In physics problems, G is the universal number that makes the gravitational equation quantitative. The experiment is one of the first ways scientists got a value for it instead of treating gravity as only a proportional relationship.
Newton's Law of Universal Gravitation
This law is the equation the Cavendish Experiment tests in real life. The force should increase with mass and decrease with the square of distance, and the apparatus is set up to detect that pull between known lead spheres. When you connect the twist of the balance to the inverse-square law, you are seeing the law as a measurable prediction.
action at a distance
Cavendish is a clean example of action at a distance because the spheres pull on each other without touching. In Physics I, that idea comes up whenever a force acts through space, like gravity or electric force. The experiment gives you a concrete lab image of a force that is transmitted without direct contact.
tidal forces
Tidal forces come from gravity acting differently on different parts of an object, and they are a natural next step after learning about Newtonian gravity. While the Cavendish setup measures the attraction between compact spheres, tidal effects explain why gravity can stretch or deform larger bodies. Both depend on gravity changing with distance.
A quiz or problem set usually asks you to identify what the torsion balance is measuring, explain why the deflection is so small, or connect the setup to Newton’s law. You might also be given a diagram and asked to label the source masses, the suspended rod, and the twisting wire, then explain how the angle of rotation relates to force. If the class includes lab work, you may need to describe the cause and effect chain: gravitational attraction creates torque, the wire twists, and the equilibrium angle lets you solve for the force or G. A good answer uses the real physics vocabulary, not just “it measures gravity.”
The Cavendish Experiment measures the tiny gravitational attraction between known masses using a torsion balance.
Its main physics trick is converting a very small force into a measurable twist in a wire.
The experiment gave scientists a way to determine the gravitational constant, G, experimentally.
It also let physicists infer Earth’s mass and density indirectly, instead of measuring the planet directly.
In Principles of Physics I, it is a real-world example of Newton’s Law of Universal Gravitation and inverse-square behavior.
It is a torsion-balance experiment that measures the gravitational attraction between small and large lead spheres. In physics, it is used to show how Newton’s law can be tested with a very small force and how G can be found from the measurement.
A thin wire or fiber suspends a rod with small masses on it. When nearby masses pull on those spheres, the rod twists, and the wire resists that twist like a spring. The angle of rotation tells you how strong the gravitational force is.
No, not directly. It measures the force between known masses in the lab, then uses that result to calculate G and infer Earth’s mass and density. That is why the experiment is often described as measuring Earth indirectly.
It gives a physical test of Newton’s Law of Universal Gravitation by showing that two masses attract each other in a way that depends on mass and distance. The setup is designed so you can use the observed deflection to connect the data back to the gravitational equation.