# derivative question

• Jan 18th 2009, 07:06 PM
billbarber
derivative question
I have a problem as follows:

An electron moving along the x axis has a position given by x=17te^-1.1t meters where t is in seconds. How far is the electron from the origin when it momentarily stops.

Now I know that I need to take the derivative of the position function to find the velocity, and then set that equal to zero. For some reason I'm having trouble figuring out the derivative of the given position function.

Any help would be greatly appreciated.
• Jan 18th 2009, 07:22 PM
gebbal
e^-1.1x (17-18.7x) maybe?
• Jan 18th 2009, 08:02 PM
Bilbo Baggins
Quote:

Originally Posted by billbarber
I have a problem as follows:

An electron moving along the x axis has a position given by x=17te^-1.1t meters where t is in seconds. How far is the electron from the origin when it momentarily stops.

Now I know that I need to take the derivative of the position function to find the velocity, and then set that equal to zero. For some reason I'm having trouble figuring out the derivative of the given position function.

Any help would be greatly appreciated.

Hello. This is the product of two functions, and you need to use the "product rule" Take the derivative of the first and multiply it by the second. Then add to that term the product of the original first function and the derivative of the second.

17e^-1.1t + -18.7te^-1.1t

Factor out the e^-1.1t and you have the product of two factors equal to zero. Therefore, one must be zero. Now, the only way to make e^-1.1t zero is to make t infinite (which yields an indeterminate form anyway here), so you can scratch that. Solve the other factor for zero. 17-18.7t=0 t=.909090
• Jan 20th 2009, 11:20 AM
billbarber
Nevermind....I think I was just too tired the other night. I woke up the next morning and I realized I just had to use the product rule. Above poster was correct about that but incorrect in his execution.

Ended up with 17e^-1.1t + 17te^-1.1t. Set that equal to zero and solved for t got t=1 and put that back into the original equation to get x (position).

Thanks to the above for the posts though.