I am trying to solve the following problem:

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> A radioactive atom is moving in the $x$ direction in the laboratory at speed $0.3c$. It emits an electron having speed $0.8c$ in the rest frame of the atom. What will be the velocity of the electron in the laboratory when it is ejected in the $y$ direction in the atom's rest frame? (Find $u\_y$, $u\_x$, $|u|$, and $\theta$.)
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I thought that since the atom is moving in the $x$ direction relative to the laboratory, then any motion in the $y$ direction relative to it must be the same for the laboratory. However, when I put $u\_y = 0.8c$, the teacher marked it wrong; he refuses to explain why. Why is $u\_y \neq 0.8c$?