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An object is moving at 0.9c toward the y-axis (with respect to reference frame S). What is the velocity of the object with respect to reference frame S' that moves at 0.9c toward the x-axis?

I get the right answer if I assume that the Lorentz factor of the object moving with respect to S' is the product of the Lorentz factor of S' and that of the object moving with respect to S. In other words

[tex] \frac{1}{\sqrt{1-\frac{x^2}{c^2}}}=\frac{1}{1-\frac{0.9^2}{c^2}} [/tex]

in which case x=0.98c which should be correct.

However, I have no idea what I'm doing here so any guidance would be appreciated.

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# Homework Help: A problem involving two velocities (special relativity)

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