Active Reading Solution: The Position

  1. Because the junction is moving at speed u, it has moved a distance ut.

  2. So at t=0 it was at position x-ut.

  3. At time 0 the number of red rulers between the origin and that junction was just the position x-ut. But the blue rulers are shortened by a factor of g. So the number of blue rulers between the origin and that junction was x'=g(x-ut).