# Homework Help: Rearranging a Relativity Equation

1. May 7, 2008

### TFM

[SOLVED] Rearranging a Relativity Equation

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

Rewrite equation:

$$f = \sqrt{\frac{c + u}{c - u}}f_0$$

to find the relative velocity u between the electromagnetic source and an observer in terms of the ratio of the observed frequency and the source frequency of light.

What relative velocity will produce a 3.0 decrease in frequency and

2. Relevant equations

Rearangement of:

$$f = \sqrt{\frac{c + u}{c - u}}f_0$$

3. The attempt at a solution

I seem to get stuck when trying to rearrange the equation:

$$f = \sqrt{\frac{c + u}{c - u}}f_0$$

$$\frac{f}{f_0} = \sqrt{\frac{c + u}{c - u}}$$

$$(\frac{f}{f_0})^2 = \frac{c + u}{c - u}$$

$$(c - u)(\frac{f}{f_0})^2 = c + u$$

But I am not sure where to go from here.

Any suggestions?

TFM

2. May 7, 2008

### Staff: Mentor

You're doing fine; don't stop now.

$$c(\frac{f}{f_0})^2 - u(\frac{f}{f_0})^2 = c + u$$

Next: Move all the terms containing u to one side.

3. May 7, 2008

### TFM

Would that give:

$$c(\frac{f}{f_0})^2 - c = u(\frac{f}{f_0})^2 + u$$

TFM

4. May 7, 2008

### TFM

Do uou now factor out the u:

$$c(\frac{f}{f_0})^2 = u ((\frac{f}{f_0})^2 + 1)$$

Tnhe divide over to get:

$$u = \frac{c(\frac{f}{f_0})^2 - c}{(\frac{f}{f_0})^2 + 1}$$

Does this look right?

TFM

5. May 7, 2008

### Staff: Mentor

Looks good! (I would factor out the c to make it more readable.)

6. May 7, 2008

### TFM

That would give:

$$u = \frac c((\frac{f}{f_0})^2) - 1}{(\frac{f}{f_0})^2 + 1}$$

What relative velocity u will produce a 3.0 % decrease in frequency

For the actual question, do I put f_0 as 1 and f as 0.97?

TFM

7. May 7, 2008

### Staff: Mentor

Redo this. (Probably a formatting error.)

8. May 7, 2008

### TFM

That definately isn't right :

$$u = \frac{c((\frac{f}{f_0})^2 - 1)}{(\frac{f}{f_0})^2 + 1}$$

TFM

9. May 7, 2008

### Staff: Mentor

Looks good.

10. May 7, 2008

### TFM

The actual question itself is:

What relative velocity u will produce a 3.0 % decrease in frequency

For this, should I put f_0 as 1 and f as 0.97?

TFM

11. May 7, 2008

### Staff: Mentor

Makes sense to me.

12. May 7, 2008

### TFM

It gives, the right answer, Thanks

One last thing, what is the question actually asking to find:

What relative velocity u will produce an increase by a factor of 3 of the observed light?

Does it want the freuqncy to be incresed by three factors?

TFM

13. May 7, 2008

### Staff: Mentor

I'm no more a mind reader than you are! But since the problem seems to be talking about frequency and Doppler shifts, I would assume they mean that the observed frequency is three times the original.

14. May 7, 2008

### TFM

I calculated it to be 0.8, which is the answer, so I should say it is, as well.

Thats the second poorly worded question I've had this week

Thanks for all the help,

TFM