Question

Newton's globe experiment: two globes that share all their features are connected with a rope in an otherwise empty universe.

Newton introduced this experiment to show that even though the cases where the globes don't move and where they rotate are not distinguishable relationally (i.e. considering their distance to each other), we can nevertheless distinguish them due to inertial forces that only occur in the rotation case (tension on the rope between the two globes).

My question: could Newton also have chosen an example using linear accelerated motion instead of angular accelerated motion? Inertial forces should occur there as well. If not, why does he need circular motion to make his point?

Answer 1

Newton's globe experiment is an illustration of inertial forces in circular motion, reflecting on how tension in the rope between rotating globes indicates the presence of inertia. While linear acceleration also involves these forces, rotational motion provided a clearer contrast for the concept and led to the development of Newton's theory of universal gravitation.

Step-by-step explanation:

Indeed, Newton could have selected an example involving linear accelerated motion to illustrate inertial forces. However, Newton's globe experiment effectively demonstrates the presence of inertial forces using angular acceleration (rotational motion), which provided a clear contrast to the relational aspect of motion without a change in distance between the two objects. In the experiment, two identical globes are connected by a string and set into rotational motion, leading to tension in the rope due to centripetal force, thus indicating the presence of inertia in a way that is readily observable.

Linear accelerated motion also involves inertial forces, as seen in Galileo's experiments with objects of different masses falling at the same rate due to gravity. However, Newton's choice of circular motion was insightful because it led to his broader understanding of celestial mechanics and gravitation, where planets and moons move in curved paths, such as ellipses, due to gravitational forces, rather than in straight lines.

The concept that gravity could act over vast distances, such as between the Earth and the Moon, helped Newton develop his theory of universal gravitation, a significant step in physics that deeply enhanced our understanding of both terrestrial and astronomical phenomenon.