Galilean Transformations

We will be using the math developed in our blog post Galilean Story.

Our goal will be to start with \overrightarrow{S} = t \overrightarrow{e}_t + x \overrightarrow{e}_x and finish with  \overrightarrow{S} = \tilde{t} \overrightarrow{e'_t} + \tilde{x} \overrightarrow{e'_x} .

The equations are shown below.

 \overrightarrow{e'}_t = \overrightarrow{e_t} + v \overrightarrow{e_x}

 \overrightarrow{e'}_x = \overrightarrow{e_x}

 \overrightarrow{e}_t = \overrightarrow{e'_t} - v \overrightarrow{e'_x}

 \overrightarrow{e}_x = \overrightarrow{e'_x}

\tilde{t} = t

\tilde{x} = -vt + x

We are going to take the equations above and start with…

\overrightarrow{S} = t \overrightarrow{e}_t + x \overrightarrow{e}_x

…and then make substitutions to slowly change the non-primed vectors and components to primed vectors and components, one equation at a time. When we are finished we will have \overrightarrow{S} as it is expressed in the primed coordinate system.

(starting equation)

 t = \tilde{t}

 x= \tilde{x} + vt

 \overrightarrow{e}_t = \overrightarrow{e'_t} - v \overrightarrow{e'_x}

 \overrightarrow{e'_x} = \overrightarrow{e}_x
\overrightarrow{S} = t \overrightarrow{e}_t + x \overrightarrow{e}_x

 \overrightarrow{S} = \tilde{t} \overrightarrow{e}_t + x \overrightarrow{e}_x

 \overrightarrow{S} = \tilde{t} \overrightarrow{e}_t + \tilde{x} \overrightarrow{e}_x + vt \overrightarrow{e}_x

 \overrightarrow{S} = \tilde{t} \overrightarrow{e'_t} - v \tilde{t}  \overrightarrow{e'_x} + \tilde{x} \overrightarrow{e}_x + vt \overrightarrow{e}_x

 \overrightarrow{S} = \tilde{t} \overrightarrow{e'_t} - v \tilde{t}   \overrightarrow{e'_x} + \tilde{x} \overrightarrow{e'_x} + vt \overrightarrow{e'_x}

When we look at the last equation…

 \overrightarrow{S} = \tilde{t} \overrightarrow{e'_t} - v \tilde{t}   \overrightarrow{e'_x} + \tilde{x} \overrightarrow{e'_x} + vt \overrightarrow{e'_x}

…we see two terms that can cancel and this simplifies it to the following …

 \overrightarrow{S} = \tilde{t} \overrightarrow{e'_t} + \tilde{x} \overrightarrow{e'_x}

We can now type what we started with being equal to what we finished with…

  t \overrightarrow{e}_t + x \overrightarrow{e}_x = \tilde{t} \overrightarrow{e'_t} + \tilde{x} \overrightarrow{e'_x}

This tells us that the vector  (t, x) is the same as  ( \tilde{t}, \tilde{x} ) as long as we use the appropriate basis set…

{ \overrightarrow{e}_t, \overrightarrow{e}_x } for the former

{ \overrightarrow{e'}_t, \overrightarrow{e'}_x } for the latter

(we have been advised that the prime marks don’t show up well, especially since a part of the arrow which is so close is very similar in its shape)

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