Frames of References (H1.1):
If you were to sit in a room, next to someone in the same room also sitting down, neither of you are moving, relative to each other. The person next to you is sitting still, he has no velocity. However this is from the frame of reference that it yourself, the observer of this 'event' is yourself. Relative to yourself, he has no velocity. However, if you were to change the frame of reference to an observer from outer space, he/she would see the entire planet as orbiting the sun as well as rotating about it's axis. As a result, both yourself and the person next to you in the room would be moving at a velocity which is equal to the orbit of the earth around the sun and its orbit about the axis of the planet, relative to the observer in outer space. Therefore velocity is not absolute when there are multiple frames of references for the same event. In one frame of reference, the velocity is zero, whereas in another frame of reference, the same event is observed to be travelling at a great velocity. The IB definition of a frame of reference: A system of coordinates that enables measurements to be made or A system of coordinates that enables the position of various objects to be specified.
Galilean Transformations (H1.2):
A Galilean transformation is a way of mathematically quantifying the velocities of two objects as they are relative to each other. If you imagine two particles travelling towards each other at 0.98c, then the relative velocities of the particles relative to each other would be the sum of their two velocities. Since they are both travelling at 0.98c, the Galilean transformation would be 1.96c. However, this is wrong when it concerns velocities greater than the speed of light, because this predicts that their relative velocities exceed the speed of light, and one of the fundamental laws of physics states that nothing can travel greater than the speed of light.
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