# Math Is Fun Forum

Discussion about math, puzzles, games and fun.   Useful symbols: ÷ × ½ √ ∞ ≠ ≤ ≥ ≈ ⇒ ± ∈ Δ θ ∴ ∑ ∫  π  -¹ ² ³ °

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## #1 2011-07-28 14:01:57

1a2b3c2212
Member
Registered: 2009-04-04
Posts: 419

### Rate of Change

A ladder AB of length 5 has one end A leaning against a vertical wall. The other end B rests on horizontal ground. When A is 4m above the ground, it slides down at 2m/s. How fast does B slide?

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## #2 2011-07-28 15:31:00

gAr
Member
Registered: 2011-01-09
Posts: 3,482

### Re: Rate of Change

Hi 1a2b3c2212,

Last edited by gAr (2011-07-28 15:33:18)

"Believe nothing, no matter where you read it, or who said it, no matter if I have said it, unless it agrees with your own reason and your own common sense"  - Buddha?

"Data! Data! Data!" he cried impatiently. "I can't make bricks without clay."

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## #3 2011-07-28 15:31:03

bobbym
bumpkin
From: Bumpkinland
Registered: 2009-04-12
Posts: 109,606

### Re: Rate of Change

Hi 1a2b3c2212;

In mathematics, you don't understand things. You just get used to them.
If it ain't broke, fix it until it is.
Always satisfy the Prime Directive of getting the right answer above all else.

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## #4 2011-07-28 15:32:45

bobbym
bumpkin
From: Bumpkinland
Registered: 2009-04-12
Posts: 109,606

### Re: Rate of Change

Hi gAr;

That is what I got before but I rejected it and recalculated so the dx / dt is positive.

In mathematics, you don't understand things. You just get used to them.
If it ain't broke, fix it until it is.
Always satisfy the Prime Directive of getting the right answer above all else.

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## #5 2011-07-28 15:34:50

gAr
Member
Registered: 2011-01-09
Posts: 3,482

### Re: Rate of Change

Hi bobbym,

The negative sign only means it's moving in the opposite direction!

"Believe nothing, no matter where you read it, or who said it, no matter if I have said it, unless it agrees with your own reason and your own common sense"  - Buddha?

"Data! Data! Data!" he cried impatiently. "I can't make bricks without clay."

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## #6 2011-07-28 15:36:40

bobbym
bumpkin
From: Bumpkinland
Registered: 2009-04-12
Posts: 109,606

### Re: Rate of Change

Hi gAr;

Isn't the ladder moving in the positive direction on the x axis as it decreases in the y direction?

In mathematics, you don't understand things. You just get used to them.
If it ain't broke, fix it until it is.
Always satisfy the Prime Directive of getting the right answer above all else.

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## #7 2011-07-28 15:39:35

gAr
Member
Registered: 2011-01-09
Posts: 3,482

### Re: Rate of Change

I think it doesn't matter, just a matter of convention.
We may consider dy/dt = -2m/s in that case.

"Believe nothing, no matter where you read it, or who said it, no matter if I have said it, unless it agrees with your own reason and your own common sense"  - Buddha?

"Data! Data! Data!" he cried impatiently. "I can't make bricks without clay."

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## #8 2011-07-28 15:51:56

bobbym
bumpkin
From: Bumpkinland
Registered: 2009-04-12
Posts: 109,606

### Re: Rate of Change

In mathematics, you don't understand things. You just get used to them.
If it ain't broke, fix it until it is.
Always satisfy the Prime Directive of getting the right answer above all else.

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## #9 2011-07-28 15:55:15

gAr
Member
Registered: 2011-01-09
Posts: 3,482

### Re: Rate of Change

Okay, I hope you got what I was trying to say.

"Believe nothing, no matter where you read it, or who said it, no matter if I have said it, unless it agrees with your own reason and your own common sense"  - Buddha?

"Data! Data! Data!" he cried impatiently. "I can't make bricks without clay."

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## #10 2011-07-28 16:03:51

bobbym
bumpkin
From: Bumpkinland
Registered: 2009-04-12
Posts: 109,606

### Re: Rate of Change

Hi gAr;

Yes I do. I am going to grab a sandwich or something, see you in a bit.

In mathematics, you don't understand things. You just get used to them.
If it ain't broke, fix it until it is.
Always satisfy the Prime Directive of getting the right answer above all else.

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## #11 2011-07-28 16:16:40

gAr
Member
Registered: 2011-01-09
Posts: 3,482

### Re: Rate of Change

Okay, see you soon.

"Believe nothing, no matter where you read it, or who said it, no matter if I have said it, unless it agrees with your own reason and your own common sense"  - Buddha?

"Data! Data! Data!" he cried impatiently. "I can't make bricks without clay."

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