A Physics Digest—Part 1: Motion

A discussion of the mechanics (the physics) of motion is pretty much a look at the early history of European physics—as it morphed from a general topic of natural philosophy into a precise science of physics. It began, however, with the Greeks. (What doesn’t, in Western culture?) A few of the Greek natural philosophers arrived at very accurate understandings of motion, while others conducted “thought experiments”—never testing out their ideas—that were shaky at best. Some of their misinterpretations hung around for nearly 2000 years, before they were fixed.

One of the key fixers was Galileo, who became one of the first natural philosophers to put his ideas to test—in 16th century Italy. He introduced the concept of inertia: that a body at rest wants to stay inactive until something budges it. More importantly, Galileo also showed us that inertia means that a moving body wants to keep in motion (at a constant speed), until something either slows it down or accelerates it. His insights paved the way for the understanding of the motion of projectiles and satellites. The moon is a satellite of Earth, as we are a satellite of the sun.

These Galilean insights opened the door for Isaac Newton to formulate his three laws of forces. He understood that a force was the thing that overcame the inertia of a body—getting it moving or slowing it down. He also discovered that every body has mass: an inherent quality that provides a measure of its inertia. Massive things have a lot of inertia. It takes more force to push them. So what is weight? It’s just a measure of how much force Earth’s gravity pulls on a body.

Newton was also the first person to grasp the essence of gravity. As I wrote in an earlier posting (5/26/09), Kepler got some hunches about gravity—believing it to be some kind of force that the sun exerts on planets—but never quite got the full picture. Newton did. The popular image has him getting his inspiration when an apple (being pulled down by gravitational force) hit him on the head. Whatever really happened, Newton understood that gravity is the force that pulls the apple and keeps us circling the sun. He came up with an elegant equation to describe it.