Lorentz transformations (2nd year relativity)

[affans]

Homework Statement

A light signal is sent from the origin of a system K at t = 0 to the point x = 1 m, y = 8 m, z = 13 m. a) At what time t is the signal received?
b) Find ( x', y', z', t' ) for the receipt of the signal in a frame K' that is moving along the x-axis of K at a speed of 0.6c.

Homework Equations

The lorentz transformations:

x' = \gamma * (x - vt)
y' = y
z' = z

The Attempt at a Solution

Part a was easy. I got the right answer. I just took the length of the vector given by the co-ordinates and divided by the speed of light. The answer is 5 * 10^8 I am having trouble with part b.

Ofcourse, y' and z' were easy to get. t' (i had 3 tries, and i used them all) so I lost a mark there. I have one try left on x'.

Using the equation, we first have to solve for \gamma. Plugging the numbers into the equation for gamma:
\frac{1}{\sqrt{1-(v/c)^2}}\; \text{yields} \; 1.25

Then using the lorentz transformation I have the following eqn:
x' = \gamma (x - vt) . Plugging in the numbers yeilds
x' = 1.25(1 - 0.6 * c *(5E-8))

I get 10 as the answer. It is wrong. I also thought t = 0 could work since that's when the event happened. But the answer 1.25 is also wrong.

My third attempt yielded 11.25m however, I am scared to submit it. If anyone can please verify my number for me.

 

[diazona]

The Lorentz transformations are more accurately written
\Delta t' = \gamma(\Delta t - v\Delta x/c^2)
\Delta x' = \gamma(\Delta x - v\Delta t)
The \Delta indicates that the numbers to be plugged in should be the difference between two spacetime events. So putting in t=0 is a mistake that you should not make again.

Can you show your calculations for the other two attempts?