# Galilean Transformation Question

1. Feb 13, 2005

### misogynisticfeminist

What happens if I have 2 frames,S and S', with S as my rest frame and S' moving in the +ve x direction (towards the right). Why is it that the equation for the galilean transformation for the x-coordinate is $$x'=x-vt$$ instead of $$x'=x+vt$$ ???

2. Feb 13, 2005

### Galileo

Think of an object lying on the positive x-axis (it therefore also lies on the x'-axis). At t=0 the x and x' coordinate of the object are the same, but since the origin in S' moves towards the object (as seen from S. In S' the object moves towards the origin.) the x' coordinate decreases with time. So x'=x-vt is the right equation.

Drawing a picture also helps.

3. Feb 13, 2005

### Staff: Mentor

The primed coordinates are the coordinates measured by the "moving" frame. Take a simple case: Imagine an object moving along with the origin of S'. After a time t, the S frame says the object's coordinates are (x,t) = (vt,t). Of course, the S' frame gives the coordinates as (x', t) = (0, t). Using $x'=x+vt$ gives the wrong answer.

4. Feb 13, 2005

### misogynisticfeminist

hey thanks alot, I've gottit, sorry if my questions are a little too simple, I'm mostly self-taught.

5. Feb 13, 2005

### dextercioby

There's no problem with us:it's essential for you to have these problems clear,especially if you want to go into studying SR,where things are a bit more difficult...

Daniel.