PHY 1371Dr. Jie Zou1 Chapter 39 Relativity

PHY 1371Dr. Jie Zou2 Outline The principle of Galilean relativity Galilean space-time transformation equations Galilean velocity transformation equation Einstein’s principle of relativity Consequences of the special theory of relativity

PHY 1371Dr. Jie Zou3 The principle of Galilean relativity Inertial frame of reference: An inertial frame of reference is one in which an object is observed to have no acceleration when no forces act on it. Any system moving with constant velocity with respect to an inertial system must also be an inertial system. Principle of Galilean relativity: The laws of mechanics must be the same in all inertial frames of reference. There is no preferred inertial reference frame.

PHY 1371Dr. Jie Zou4 Galilean space-time transformation equations Event: Some physical phenomenon that occurs in an inertial system. Space-time coordinates of an event: the four coordinates (x,y,z,t). Galilean space-time transformation equations: x’ = x – vt, y’ = y, z’ = z, t’ = t. Assumption: Time is assumed to be the same in both inertial systems. That is, within the framework of classical mechanics, all clocks run at the same rate, regardless of their velocity. Consequently, the time interval between two successive events should be the same for both observers. This assumption turns out to be incorrect in situations where v is comparable to the speed of light.

PHY 1371Dr. Jie Zou5 Galilean velocity transformation equation Now suppose that a particle moves a distance dx in a time interval dt as measured by an observer in S. Galilean velocity transformation equation: u x ’ = u x –v Here, u x and u x ’ are the x- components of the velocity relative to S and S’, respectively.

PHY 1371Dr. Jie Zou6 Quick quiz 39.2 Applying the Galilean velocity transformation equation, determine how fast (relative to the Earth) a baseball pitcher with a 90-mi/h fastball can throw a ball while standing in a boxcar moving at 110 mi/h.

PHY 1371Dr. Jie Zou7 Einstein’s principle of relativity Einstein based his special theory of relativity on two postulates: 1. The principle of relativity: The laws of physics must be the same in all inertial reference frames. 2. The constancy of speed of light: The speed of light in vacuum has the same value, c = 3.00 x 10 8 m/s, in all inertial frames, regardless of the velocity of the observer or the velocity of the source emitting the light.

PHY 1371Dr. Jie Zou8 Consequences of the special theory of relativity The concepts of simultaneity, time, and length are quite different in relativistic mechanics from what they are in Newtonian mechanics. In relativistic mechanics there is no such thing as absolute length or absolute time. Events at different locations that are observed to occur simultaneously in one frame are not observed to be simultaneous in another frame moving uniformly past the first.

PHY 1371Dr. Jie Zou9 Simultaneity and the relativity of time A thought experiment. Two events that are simultaneous in one reference frame are in general not simultaneous in a second frame moving relative to the first. That is, simultaneity is not an absolute concept but rather one that depends on the state of motion of the observer.

PHY 1371Dr. Jie Zou10 Homework Ch. 39, P. 1277, Problems: #1, 2.