Special Relativity: Note 1

Introduction to Modern Physics & Frames of Reference From Classical to Modern Physics By the end of the 19th century (1800’s), physicists thought they had accomplished almost all understanding of the physical world. • Newton’s laws described motion of objects on Earth and in the Heavens -Everything from the motion to a thrown ball to the orbit of the moon could be explained. -Electromagnetism was well understood. Light was mostly considered to be an electromagnetic wave. -Thermodynamics was fully described by its own laws. • Remaining puzzles: Structure of the atom & the orbit of Mercury. -Many scientists still regarded the atom as a solid “billiard ball” or “raisin bun.” -Mercury’s orbit slowly perturbed every rotation. -At this time, two revolutionary ideas began to surface to solve these problems: 1. Quantum Mechanics. 2. Relativity. Frames of Reference -An arbitrary origin from which to view motion. 1. Inertial Frames The Law of Inertia holds in any inertial frame. That is “if no net force acts on an object at rest, it remains at rest, or if in motion, it continues to move in a straight line at constant speed.” Eg. Constant velocity of a train, car, boat, space ship. A house, cat, etc. at rest. 2. Accelerating Frames of Reference: An accelerating frame of reference is a non-inertial frame. That is, the laws of Newtonian Mechanics DO NOT apply! Eg. Accelerometer. This is like some fuzzy dice hanging on a rear view mirror. Thought Experiment: Say the classroom was a space ship with no windows. How could we tell if it was at rest or moving at constant velocity? Due to different frames of reference, any observer can always assume that they are at rest, while everything else is in motion relative to them. This allows any observer to only be able to absolutely measure two speeds: 1. 2.