Modern Physics question -- an atom ejecting a relativistic electron

[Dan350]

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?

 

[PeroK]

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!

 

[Dan350]

You just have to sort out the minus sign!

So the awnser is simply 0.4c?

 

[PeroK]

So the awnser is simply 0.4c?

What you didn't do was set the problem up in any meaningful way. Why did you use the equation you did? Can you not check an answer of 0.4c yourself?