Rate Of Change (derivatives) Word Problem

  • Thread starter nexxia
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  • #1
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Homework Statement



A 1.8 m tall student is trying to escape from the minimum security prison in To no.
She runs in a straight line towards the prison wall at a speed of 4.0 m/s. The guards
shine a spotlight on the prisoner as she begins to run. The spotlight is located on
the ground 30 m from the wall. At which rate is the prisoner's shadow on the wall
decreasing when she is 20 m from the wall?


Homework Equations


All basic differentiation rules.


The Attempt at a Solution


This is my problem, I know how to do derivatives fine, I can't set up the equation based on the information given, I was wondering if anyone could give me clues on how to set it up?
 

Answers and Replies

  • #2
tiny-tim
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welcome to pf!

hi nexxia! welcome to pf! :wink:
I know how to do derivatives fine, I can't set up the equation based on the information given, I was wondering if anyone could give me clues on how to set it up?
first you need to write out the equations for where everything is at a fixed time t …

what do you get? :smile:
 
  • #3
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hi nexxia! welcome to pf! :wink:


first you need to write out the equations for where everything is at a fixed time t …

what do you get? :smile:
Do you mean manipulating v=d/t to make things equal to t and then make them equal to eachother?
going out on a limb here;
like t=d/v= 20m/4m/s ???
 
  • #4
tiny-tim
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Do you mean manipulating v=d/t to make things equal to t and then make them equal to eachother?
going out on a limb here;
like t=d/v= 20m/4m/s ???
uhh? :confused:

just write out the equations for where the prisoner is, and where the shadow is, at any fixed time t :redface:
 
  • #5
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uhh? :confused:

just write out the equations for where the prisoner is, and where the shadow is, at any fixed time t :redface:
I dont know how do do that :confused:
 
  • #6
tiny-tim
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start with …
She runs in a straight line towards the prison wall at a speed of 4.0 m/s.
… convert that from English into an equation
 

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