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Old Nov 12th 2010, 11:08 AM   #1
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Mirror Question

Standing 2.5 m in front of a plane mirror with your camera, you decide to take a picture of yourself without turning the camera around. To what distance should the camera be focused to get a sharp image?

Is there an equation for mirror questions like this?
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Old Nov 12th 2010, 02:27 PM   #2
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2.5m to the mirror
the image is formed behind the mirror 2.5m

so 2.5*2
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Old Nov 13th 2010, 12:46 PM   #3
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Originally Posted by Haytham View Post
2.5m to the mirror
the image is formed behind the mirror 2.5m

so 2.5*2
Is it really just that simple? Wow, thanks though I should have just thought it through a little more!

Anyways, here's another question but with velocity this time.

"A dog is sitting in front of a plane mirror. At a command it jumps at the mirror with a speed of 0.50 m/s. How fast does the dog approach its image?"

How would you do that one?
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Old Nov 13th 2010, 03:37 PM   #4
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Originally Posted by jrodd321 View Post
"A dog is sitting in front of a plane mirror. At a command it jumps at the mirror with a speed of 0.50 m/s. How fast does the dog approach its image?"

How would you do that one?
If the dog moves 1 m toward the mirror, what do you think the image will do?
Can you take it from there?
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Old Nov 13th 2010, 09:31 PM   #5
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Originally Posted by arbolis View Post
If the dog moves 1 m toward the mirror, what do you think the image will do?
Can you take it from there?
It will get larger? I'm really confused in this question. I understood the first one I asked where you are 2.5 away and the image is formed 2.5 meters behind the mirror so it would be 5. But I don't understand how you would know the dogs speed if it just tells you how he starts out and doesn't give his distance away from the mirror initially.
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Old Nov 14th 2010, 01:39 AM   #6
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Take it for one second. If the dog made 0.5 m in one second towards the mirror, how far would the image travel during the same time?

From this, you should be able to deduce the speed of the image towards the surface of the mirror, and hence the speed of approach with respect to the dog.
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Old Nov 14th 2010, 08:00 AM   #7
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Originally Posted by Unknown008 View Post
Take it for one second. If the dog made 0.5 m in one second towards the mirror, how far would the image travel during the same time?

From this, you should be able to deduce the speed of the image towards the surface of the mirror, and hence the speed of approach with respect to the dog.
It would travel the same speed. 0.5 m/s right?
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Old Nov 14th 2010, 08:06 AM   #8
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Right.

Now, the dog approaches the mirror with a speed of 0.5 m/s and the image approaches the mirror with a speed of 0.5 m/s. How fast does the image approach the dog?
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Old Nov 14th 2010, 10:37 AM   #9
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Originally Posted by Unknown008 View Post
Right.

Now, the dog approaches the mirror with a speed of 0.5 m/s and the image approaches the mirror with a speed of 0.5 m/s. How fast does the image approach the dog?
It approaches the dog at 1 m/s?
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Old Nov 14th 2010, 10:41 AM   #10
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Right! Well done

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