Inertial Frames of Reference and very Special Relativity

People always tell you to consider things from different perspectives because the same thing can look different to different observers.

Einstein proved this mathematically with Special Relativity! Isn't physics awesome?

Okay so first, learn that an inertial frame of reference is one in which Newton's Law of Motion apply. The two conditions neccesary for an intertial referece are constant speed or zero speed. An inertial frame of reference has no acceleration.

Dropping a ball to measure g in a static lab yields the same result as doing the experiment on a train moving at constant speed.

If speed is changing - this means there is accelration. Accelerated frames of reference are non inertial frames. In circular motion, velocity is changing and the body is accelerated towards the center, so a rotating frame is a good example for a non inertial frame of reference. Although the speed is the same, velocity is changing (due to its vector nature).

Lets build on that. The Two Postulates of Relativity state that:

1.) The Law of Physics are the same in all inertial frames of reference. If you are on a car at 30km/h, you can assert that you are travelling at 30km/h relative to the ground (so the ground is the inertial frame of reference). However you would be stationary relative to the car (this time the car is the inertial frame of reference).

2.) The speed of light is the same to all observers. The speed of light is measured relative to the observer and is a constant defined by Maxwell's equation.

Simultaneity can be explained using Special Relativity. If two events happen simultaneously, they are called simultaneous events.

Watch video:



The video is trying to say that two event CANNOT be simulateous in the two inertial frames of reference. The lady saw the flashes happen one before the other while the external observer saw both happen at the same time, the two observations are correct according to special relativity.

IB questions have the same style to simultaneous questions. You get marks for saying:

1.) No simultaneity can occur between the two inertial frames of reference. So if A sees something happening at the same time, then B will see them happening one before the other and vice versa.
2.) Speed of light is constant in all frames of reference
3.) The tricky part is determining which event happens first.