Modern Physics question -- an atom ejecting a relativistic electron

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Dan350
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Homework Statement

.
An atom at rest can undergo radioactive decay, ejecting an electron at a maximum speed of 0.5c. If the atom in a particle accelerator is observed to produce an electron traveling at 0.75c, at least how
fast must the atom itself have been moving?

Homework Equations


u0 x =(ux −v )/(1−uxv/c^2)

The Attempt at a Solution


The problem is asking for the speed of the atom,

I set
ux=0.5c
v=0.75
After plugin in, I get the result of -0.4c

I don't feel sure aboout this anwser.
Any suggeestions? Corrections?
 
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Dan350 said:

Homework Statement

.
An atom at rest can undergo radioactive decay, ejecting an electron at a maximum speed of 0.5c. If the atom in a particle accelerator is observed to produce an electron traveling at 0.75c, at least how
fast must the atom itself have been moving?

Homework Equations


u0 x =(ux −v )/(1−uxv/c^2)

The Attempt at a Solution


The problem is asking for the speed of the atom,

I set
ux=0.5c
v=0.75
After plugin in, I get the result of -0.4c

I don't feel sure aboout this anwser.
Any suggeestions? Corrections?

You just have to sort out the minus sign!
 
PeroK said:
You just have to sort out the minus sign!
So the awnser is simply 0.4c?