# Homework Help: Modern Physics question -- an atom ejecting a relativistic electron

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1. Sep 27, 2016

### Dan350

1. The problem statement, all variables and given/known data.
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?

2. Relevant equations
u0 x =(ux −v )/(1−uxv/c^2)

3. 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?

Last edited by a moderator: Sep 27, 2016
2. Sep 27, 2016

### PeroK

You just have to sort out the minus sign!

3. Sep 27, 2016

### Dan350

So the awnser is simply 0.4c?

4. Sep 28, 2016

### PeroK

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?