"The laws of physics are the same in any inertial frame of reference."

These physical laws help us to understand how and why our environment reacts the way it does. They also allow us to predict events and their outcomes. As a result of this postulate, we can formulate such laws and be sure that they are independent of our current state of motion. Consider a yardstick and a cardboard box. If you measure the length of the box, you will obtain the same result whether you are standing on the ground or holding the box while riding on a bus moving at a constant velocity.


"The speed of light (c) is the same for all observers no matter what their relative speeds. In a vaccum, this speed is 300 000 000 m/s approximately."

Both observers measure the speed of light to be the same15.

In other words, the speed of light is a fundamental constant of nature. Suppose I am in a car going 50 km/h, and I throw a baseball 10 km/h in the same direction the car is going. If you were standing on the side of the road with a radar gun, you’d measure the baseball going 50 + 10 = 60 km/h. That’s how we classically deal with relative motion. Now suppose instead I am in the car shining a flashlight. If you could measure the speed of the light coming out of it from the side of the road, you’d get the same speed no matter fast the car was going, or what direction I shined the light. That is what makes light special!

The consequences of these postulates are described in the following sections:
Length Contraction
Time Dilation
Doppler Effect
Mass Increase
Energy of Light Rays


A train is traveling by you at 0.8c. This train fires out a light beam straight ahead.
You measure the speed of this light beam to be:

a. c
b. less than c
c. greater than c

The speed of light is:

a. 300 000 000 m/s
b. 300 m/s
c. 300 000 m/s

Inertial Reference Frames Length Contraction