Physics 101- Introduction to Mechanics
January 30- Constant Acceleration Motion






  Special Relativity (see chapter 39 for more details)
Einstein thought about motion on a very fast train train and realized that you shouldnít be able to have anything go faster than the speed of light.  He stated two assumptions-
  • The laws of physics are the same in all inertial reference frames.  (As long as youíre not accelerating, any experiment you perform will yield the same result.)
  • The speed of light is fixed.  (No matter the speed of the source or observer, light travels with the same speed.)

From these two assumptions you quickly uncover oddities* such as time dilation, length contraction, and various paradoxes. 

*Oddities means things we donít experience in our daily lives when travelling at speeds much less than 3 x 108 m/s.)

The core issue is that observers in different reference frames can no longer agree what are simultaneous events.

Length contraction:
When viewed from a different reference frame, a moving object will appear shorter to an observer.  This isn't an optical illusion, this is real- one person would be correct to say that an object is 1m long, and a second would also be correct to say that the same object is only 0.30m long.  Both are correct!

An object travelling at 10% the speed of light:

An object travelling at 86% the speed of light:

An object travelling at 99% the speed of light:


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Jeff Phillips
Loyola Marymount University
Spring 2002