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Grade: 12th Pass


A man of height 1.8m walks away from a lamp at a height of 6m. if the man’s speed is 7m/s , the speed in m/s at which the tip of the shadow moves is A)5m/s B)7m/s C)10m/s D)17m/s

3 years ago

Answers : (1)

Vikas TU
12280 Points
The shadow triangle and the men’s triangle with lamp would be simlar.
Hence let x be the distance of tip of the shadow from man and y be the distance of man from the lamp at any instant.
from similar triangles we get,
1.8/x = 6(x+y)
7x = 3y........................................................(1)
Now from distance – speed formulae:
for man,
7 = y/t => t = y/7 seconds.
for shadow,
v = (x+y)/t => t = (x+y)/v
now time would be same for both therfore,
v = 7x/y + 7 
from eqn. (1) we get,
v = 3 + 7 => 10 m/s.
3 years ago
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