# Isaac Newton, Henry Cavendish and Universal Gravity

**Experiment by Henry Cavendish**

The most notable endeavour in estimating the value of the gravitational constant was an experiment conducted two centuries ago by the English chemist Henry Cavendish. Cavendish, born in 1731 into a wealthy British family, studied at the University of Cambridge. Thereafter, he spent over fifty years in London as a scientist.

One of Cavendish’s most famous experiments was the torsion balance experiment he conducted in 1798 to measure the value of the gravitational constant. His apparatus consisted of a light, rigid road about 2 feet long. He attached two small lead spheres to the ends of the rod and suspended it by a thin wire. Cavendish then brought two large lead spheres near the smaller spheres. Large spheres exerted a gravitational force on the smaller spheres, and it twisted the rod to a measurable angle. Cavendish measured the angle. In this way, he discovered a very small torsion force felt over the wire.

Through this experiment, Cavendish was able to measure all four components of Newton’s Universal Force of Gravity equation. Thus, for the first time, the value of the gravitational constant was determined. The value of ‘G’ that Cavendish calculated at that time was 6.75 x 10-11 N m2/kg2 . Cavendish first used this unit to measure the mass of the Earth which he calculated to be approximately six trillion trillion kilograms (5.97219 × 10 24 kilograms). That discovery was a milestone in the field of geophysical science.

In 1998, 45 physicists gathered in London to celebrate the 200th anniversary of Cavendish’s torsion balance experiment. On this occasion, they reviewed and evaluated many experiments designed to calculate the gravitational constant.

Across experiments, the value of the gravitational constant was not very different from Cavendish's result.

**Difference between weight and mass**

The words weight and mass are often used interchangeably. However, in reality, these are two completely different units of measurement. Your mass is the same wherever you are in the universe but your weight changes from place to place. Mass is measured in kilograms even though we usually talk about weight in kilograms. Weight is actually measured in Newtons which is the unit of force. This is because weight is the downward force that gravity exerts on an object. This force increases with an objects' mass.

Newton’s greatest contribution to the world was the realisation that the universe is a place based on deep mathematical principles.

Mass is a measure of how much matter is contained within an object. The SI unit for mass is the kilogram. The mass of an object can be found by applying on it a known force F and dividing the size of that force by the resulting acceleration. Mass is different from weight in that its value does not change when gravity changes.

Suppose there is a person who has a mass of 60 kilograms on earth. If we use Newton's equations, we can calculate that the same person would weigh 588 Newtons on earth. Whereas on the moon, where the acceleration due to gravity is about one-sixth of what it is on earth, that same person would weigh only 98 Newtons.

Newton’s greatest contribution to the world was the realisation that the universe is a place based on deep mathematical principles. At that time, it was a very modern and innovative thought. It completely changed the mindset of later generations. Renowned scientific historian Bernard Cohen says that the greatest achievement of Newtonian science was that it was possible to explain the whole universe for the first time based on the principles of motion.

Now put on your thinking hats and think about the following questions for a couple of minutes.

As a teacher, how do you describe Cavendish's "torsion-balance experiment" to your students?

Can you think of the ways in which mass differs from weight?

How would you describe the contributions of Henry Cavendish in finding the value of gravitational constant?

Write down your thoughts and discuss them with your students, children and your colleagues. Listen to their views and compare them with your own. As you listen to others, note how similar or different your views are to others’.

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