SOLUTION: A person of height 1.68 m is walking away from a lamp-post at 1.5 m/s. The light on the lamp-post is 5.3 m above the ground. At what rate (in m/s to the nearest cm/s) is the le

Question 989420: A person of height 1.68 m is walking away from a lamp-post at 1.5 m/s. The light on the lamp-post is 5.3 m above the ground.
At what rate (in m/s to the nearest cm/s) is the length of the person's shadow changing?
s/5.3 = (s-p)/1.68
1.68 s = 5.3 s - 5.3 p
3.62 s = 5.3 p
s = 1.464 p
ds/dt = 1.464 dp/dt
but dp/dt = 1.5
so
ds/dt = 2.196 m/s
This answer was incorrect, I am not sure where I went wrong?
The next part of the question:
At what speed (in m/s to the nearest cm/s) is the tip of the person's shadow moving across the ground?
And I assume to get this, I take the answer from the first part, and add 1.5?
I have tried many ways but keep getting the first part incorrect
THANK YOU
Answer by thesvw(77) (Show Source): You can put this solution on YOUR website!
It took me sometime to understand what went wrong. Your answer represents the rate of changing S which is not the mans shadow. It is the addition off mans shadow and the distance man went from the post. After little calculations I got this expression, s-p= 1.68*p/(5.3-1.68)
You will get it. You just misunderstood.
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