# Rate of change in distance question

1. Apr 29, 2008

### Dr Zoidburg

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

$$z^{2}$$ - $$y^{2}$$= $$x^{2}$$
differentiate with respect to time:
d/dz$$z^{2}$$ - d/dy$$y^{2}$$= 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. Apr 29, 2008

### tiny-tim

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
doesn't make sense, does it?

3. Apr 29, 2008

### Dr Zoidburg

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. Apr 29, 2008

### tiny-tim

I thought Dr Zoidburg scuttled ?

5. Apr 29, 2008

### Dr Zoidburg

whoops. you're right there. scuttled. whoop! whoop! whoop!

"Friends, help! A guinea pig tricked me!"

6. Apr 30, 2008

### tiny-tim

I just did a google search for "A guinea pig tricked me",

and got 5370 hits!