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Rate of change in distance question 
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#1
Apr2908, 02:40 AM

P: 39

Okay, I'm 99% sure I've got the right answer here, but I just wanted to make certain before I send my assignment in. It's the last question and has been bugging me for the last few days until I had an eureka moment just a few minutes back.
(In case you're wondering, I'm doing my studies by correspondence, so other than course notes and textbooks borrowed from the library I have just the internet and my brains (hah!) to aid me) 1. The problem statement, all variables and given/known data A baseball diamond has sides 27m long. A player is running from 2nd to 3rd at a speed of 9m/s. When he is 6m away from 3rd, at what rate is the player's distance from home plate changing at that instant? 3. The attempt at a solution x = distance from home plate to 3rd = 27m y = distance from player to 3rd = 6m z = distance from player to home = 27.66m (using pythagoras) speed of player is change of y over time: dy/dt = 9m/s [tex]z^{2}[/tex]  [tex]y^{2}[/tex]= [tex]x^{2}[/tex] differentiate with respect to time: d/dz[tex]z^{2}[/tex]  d/dy[tex]y^{2}[/tex]= 0 (since x doesn't change over time) dz/dt*2z  dy/dt*2y = 0 divide by 2: z*dz/dt  y*dy/dt = 0 sub the above (z, y, dy/dt) into the equation and solve: dz/dt = 1.95m/s If this ain't correct, please tell me quickly as I need to post my assignment off asap! 


#2
Apr2908, 03:07 AM

Sci Advisor
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P: 26,160

Hi Dr Zoidburg!
Yes, that's fine (but a little messy)! Try shortening it a bit. For example, there's no need to define an x (I know it's useful for helping you get to your eureka moment, but once you're there, you can forget it) … just say z² = y² + 729 (or z = √(y² + 729)). And 


#3
Apr2908, 06:19 AM

P: 39

yay, got it right! Off to the post office I scurry.
And that other bit just came out poorly due to bad formating. It looks better in my assignment 


#4
Apr2908, 06:23 AM

Sci Advisor
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P: 26,160

Rate of change in distance question
I thought Dr Zoidburg scuttled ?



#5
Apr2908, 08:38 PM

P: 39

whoops. you're right there. scuttled. whoop! whoop! whoop!
"Friends, help! A guinea pig tricked me!" 


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