By the end of this sections students should be able to:
·       outline the nature of inertial frames of reference

·       perform an investigation to help distinguish between non-inertial and inertial frames of reference

Frames of reference

Before considering the difference between inertial and non-inertial frames of reference it is firstly important to consider what is a frame of reference and why is it important to understanding motion.  A frame of reference is most easily defined as being that which other things are measured from.  That means, for example, that we only know how fast we are going because we are able to compare it to those things around us.  Consider the following, if a person is standing on the surface of the Earth next to a large tree, how fast are they moving?  Most people would respond that they are not moving, that is because they have a velocity of zero relative to the tree and surrounds.  However, they are actually moving relative to other frames of reference.  For example they could be argued to be moving at nearly 460 m/s relative to the core of the Earth (this is the speed of the Earth's rotation), or they could be said to be travelling at over 170,000 km/h relative to the Sun (that is the speed of the Earth's orbit).  So the original question is missing something about what frame of reference the question is considering.

The experience of relativity is often felt when you are on a train and your sole vision is of another train.  You sometimes feel that you are moving, despite being stationary relative to the station, as the other train begins to leave.  In such a situation there are two important components to considering what is happening.  Firstly, who and where is the observer (O) and what frame or reference (R) are they referring to?

Watch the following video and consider are you, as the observer, moving or is the train that you are looking at moving?  How do you know which is correct?

Inertial Frames of Reference

In the train example in the video above the observer, that is the person taking the video, and the train are within an inertial frame of reference, but what does this mean? An inertial frame of reference is that which has a constant velocity, or more specifically where Newton's First Law of Motion readily applies.  Consider this, you are in a car and throwing a ball up and down in your hand.  If the car is moving at a constant velocity then the ball will come back into your hand.  However, if the car is accelerating then the ball will end up in you lap, or if the car stops suddenly then the ball will land in front of you.  Where the car is moving at a constant velocity you are said to be in an inertial frame of reference as everything within this frame of reference is moving at the same speed.  Therefore if you placed the ball on the seat next to you it will stay there until a force acted upon it.  If it was a non-inertial frame of reference, that is the car is accelerating, then the ball will move when placed on the seat without a force, from within the frame of reference, acting upon it.  The Earth is an inertial frame of reference because it is moving about the Sun and it axis at a constant velocity.

Galileo first explained this idea when he proposed the Earth to be moving around the Sun (note here that he was not the first to do this).  He was challenged that if the Earth was moving, as he stated, and a bird was in a tree and wanted to reach a worm in the ground, as the bird flew from the tree the worm in the ground would move and be missed by the bird.  Likewise if we jumped we would end up landing in a different place to where we began from.

TASK: Consider the above ideas and present an argument as to why you land in the same spot when you jump, and why the bird can fly to the worm.

Galileo also concluded that it was impossible to determine the speed of a frame of reference from within the frame of reference.  That is for the example of the car above we can not know how fast the car is moving by observing those objects within the car.  This is because if the car is stationary or moving at a constant velocity the same effects will be seen.  Instead to measure the speed of a frame of reference we must observe it relative to another frame of reference.  This is the underlying principle of relativity.