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## #1 2006-01-25 10:00:18

mb1985
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### product rule problem....

A electron moving along the x axis has a position given by x = 24 te^-2.2 t m, where t is in seconds. How far is the electron from the origin when it momentarily stops?

i belive im supposed to use the product rule. but i cannot seem to get the correct answer.
can some please walk me through it. thanks in advance.

## #2 2006-01-25 11:25:35

Ricky
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### Re: product rule problem....

i belive im supposed to use the product rule. but i cannot seem to get the correct answer.

Always post all your work.  Not only does it make it easier for us, but it shows us exactly where you went wrong.

How far is the electron from the origin when it momentarily stops?

You need to find out when it stops, aka when velocity is 0.  So to find an equation for speed, you need to take the derivative of the distance (location).

It sounded like you already knew that, so what are you having trouble with?  Just taking the derivative?

"In the real world, this would be a problem.  But in mathematics, we can just define a place where this problem doesn't exist.  So we'll go ahead and do that now..."

## #3 2006-01-25 12:42:41

mb1985
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### Re: product rule problem....

yeah. bc im supposed to solve for t.

is the 24 supposed to be used as a constant? or when i use product rule do i just use the 24t by itself for the product rule?

24(t x e^-2.2t  + ( e^-2.2t x 1)

what do i do with the -2.2?? do i bring that down in front of the e. so it would be -2.2e^-t?

im just confused.

## #4 2006-01-25 23:57:21

mathsyperson
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### Re: product rule problem....

#### mb1985 wrote:

is the 24 supposed to be used as a constant? or when i use product rule do i just use the 24t by itself for the product rule?

It doesn't matter. Either way will give you the same answer.

#### mb1985 wrote:

what do i do with the -2.2?? do i bring that down in front of the e. so it would be -2.2e^-t?

Yes, but you also leave it where it was, so the derivative is -2.2e^-2.2t

And from there you just have to equate it to 0. Feel free to come back if you're still stuck.

Why did the vector cross the road?
It wanted to be normal.

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