Concave Mirror Homework - Find Object Dist.

[Boxter]

Homework Statement

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

Homework Equations

1/u + 1/v = 1/f
m = v/u

The Attempt at a Solution

f = 20cm
u = ?
v = ?

 

[Redbelly98]

Welcome to Physics Forums.

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

 

[Boxter]

Welcome to Physics Forums.

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

Sorry, but I still don't understand

 

[Redbelly98]

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.

 

[donkeycopter]

Is the object <answer deleted by moderator> away?

 

[Boxter]

I have the answers on the back of the book but can't figure out how to get it

 

[Redbelly98]

And I will try one more hint. You wrote this equation earlier:

m = v/u

What is the value of the magnification m, based on the information given in the problem statement?

 

[Boxter]

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?

Magnification is 3 ??

 

[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?

 

[Redbelly98]

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?

Uh, I don't think so. You get the answer by working with the two equations you wrote earlier.

Magnification is 3 ??

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.)