F.3 Physics (longman 3M2)

Kori

2010-11-27 4:47 am

p.11912a
Sam is looking at a clock hung on a wall through a plane mirror of height 0.4m as shown below. His eyes are 1.2m above the ground and 1 m away from the mirror horizontally. The distance between the mirror and the wall behind Sam is 4 m. The bottom of the clock is 2.4m above the floor and its length is 0.4m.

The solution is:
Let h be the maximum distance between the lower edge of the mirror and the ground.By proportion,

(h-1.2)/(2.4-1.2)=1/(1+4)
h=1.44
The maximum distance between the lower edge of the mirror and the ground is 1.44 m .

I don't understand why the solution is like this. Can you explain how the solution comes from?

回答 (1)

天同

2010-11-28 8:20 am

✔ 最佳答案

Since the distance between the mirror and the wall is given to be 4m, the image of the wall is thus 4 m behind the mirror. Draw a straight line linking the lower edge of the clock image, passing through the lower edge of the mirror, and ending at the eye of Sam.

From Sam' eye, then draw a hotizontal striaght line, passing through the wall where the mirror is, and end at the image of the wall where the clock is. You would have two similar triangles. By equating the ratio of corresponding sides of the two similar triangles, you could find,(h-1.2)/(2.4-1.2) = 1/(4+1)
from which you can solve for h

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