Asked by uwishin

A street lamp is 6m above a straight road.A
Man 2m tall.walks along the road away from
the lamp at a constant speed of 1.5m/s.At what
rate is his shadow lengthening?

Answers

Answered by Damon
Well, it depends where he is I think
at the beginning
x = man from light pole= Xi
then
x = Xi + 1.5 t
if he starts at the pole at t = 0 then Xi = 0 but we can not assume that

y = shadow length=(2/6)(x+y)
or
y = x/2

so
y=(1/2)(Xi + 1.5 t)
and
dy/dt = (1/2)(1.5) = 3/4 m/s

Answered by Steve
Draw a diagram. Using similar triangles, if

x = the distance from the lamp post
s = length of shadow

s/1.5 = (x+s)/6
s = x/3

so,

ds/dt = 1/3 dx/dt
...

Extra credit: how fast is the tip of the shadow moving?
Answered by Reiny
Using Steve's definitions
by similar triangles:
2/s = 6/(s+x)
6s = 2s + 2x
4s = 2x
2s = x
2 ds/dt = dx/dt
ds/dt = 1.5/2 = 3/4 or .75 m/s

which is what Damon had. I think Steve misread the man's speed as his distance.

Notice the speed of the shadow is independent of where the man is walking.
Answered by Steve
Reiny is correct.
Answered by shankar
Can you explain me about similar triangle concept.?
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