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RelativityRelativity

H1: Introduction to relativityH1: Introduction to relativity

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Motion is relativeMotion is relative Whenever we talk about motion, we must Whenever we talk about motion, we must

always specify the vantage point from which always specify the vantage point from which motion is being observed and measured.motion is being observed and measured.

To measure the speed of an object, we first To measure the speed of an object, we first choose a frame of reference and pretend that choose a frame of reference and pretend that we are in that frame of reference standing we are in that frame of reference standing still. still.

Then we measure the speed with which the Then we measure the speed with which the object moves relative to us-that is, relative to object moves relative to us-that is, relative to the frame of reference. the frame of reference.

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All laws of mechanics remain the same in a All laws of mechanics remain the same in a frame moving at a constant velocity.frame moving at a constant velocity.

In other words: Newton's laws can be used In other words: Newton's laws can be used in uniformly moving frames, as if those in uniformly moving frames, as if those frames did not move. The theory of frames did not move. The theory of relativity modifies those laws when the relativity modifies those laws when the magnitude of v or v0 approaches the speed magnitude of v or v0 approaches the speed of light in vacuum, but all motions studied of light in vacuum, but all motions studied here are much slower.here are much slower.

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Galilean transformationGalilean transformation

((mechanicsmechanics) A mathematical ) A mathematical transformation used to relate the space transformation used to relate the space and time variables of two uniformly and time variables of two uniformly moving (inertial) reference systems in moving (inertial) reference systems in non-relativistic kinematics. non-relativistic kinematics.

We use Galilean transformations when We use Galilean transformations when objects are not accelerating objects are not accelerating

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Galilean transformationsGalilean transformations

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Postulates of the Special Postulates of the Special Theory of RelativityTheory of Relativity

All motion is relative, not to any stationary All motion is relative, not to any stationary hitching post in the universe, but to arbitrary hitching post in the universe, but to arbitrary frames of reference.frames of reference.

A familiar experience to a passenger on a train A familiar experience to a passenger on a train who looks out his window and sees the train on who looks out his window and sees the train on the next track moving by his window.the next track moving by his window.

The important point: If you were in a train with The important point: If you were in a train with no windows, there would be no way to no windows, there would be no way to determine whether the train was moving with determine whether the train was moving with uniform velocity or was at rest. uniform velocity or was at rest.