# Homework Help: Concave Mirrors

1. Sep 9, 2010

### Boxter

1. The problem statement, all variables and given/known data
A concave mirror of focal length 20cm forms an image which is three times the height of the object. Find the distance of the object from the mirror if: 1: The image is real
2: The image is virtual

2. Relevant equations
1/u + 1/v = 1/f
m = v/u

3. The attempt at a solution
f = 20cm
u = ?
v = ?

2. Sep 9, 2010

### Redbelly98

Staff Emeritus
Welcome to Physics Forums.

The magnification tells you the ratio v/u, though it could be either positive or negative.

3. Sep 10, 2010

### Boxter

Sorry, but I still don't understand

4. Sep 10, 2010

### Redbelly98

Staff Emeritus
Okay, think about it this way: based on the given information, we can say that
v = ____ × u
where you must fill in the blank with a number.

5. Sep 10, 2010

### donkeycopter

Is the object <answer deleted by moderator> away?

Last edited by a moderator: Sep 10, 2010
6. Sep 11, 2010

### Boxter

I have the answers on the back of the book but cant figure out how to get it

7. Sep 11, 2010

### Redbelly98

Staff Emeritus
And I will try one more hint. You wrote this equation earlier:
What is the value of the magnification m, based on the information given in the problem statement?

8. Sep 11, 2010

### Boxter

Magnification is 3 ??

9. Sep 11, 2010

### Boxter

If I divide 3 into 20 I get 6.666667. If I add 6.6667 to 20, is that how I get the real height, then take 6.66667 from 20 to get virtual height?

10. Sep 11, 2010

### Redbelly98

Staff Emeritus
Uh, I don't think so. You get the answer by working with the two equations you wrote earlier.
Yes, good. So your two equations your wrote in Post #1 become:

1/u + 1/v = 1/f
___ = v/u​

(Fill in the blank to get the complete equation.)