# Physics/Calculus problem

• Feb 25th 2010, 09:13 PM
Zanderist
Physics/Calculus problem
http://www.mathhelpforum.com/math-he...1&d=1267164367

I know how to begin, take the derivative.

okay that comes out to be around

"5t^4-24t^3"

So I factor out a "t"

"t^3(5t-24)"

then I set equal to "0"

"5t-24=0"

and the answer should be "24/5" but it's wrong. What have I done wrong.
• Feb 25th 2010, 09:21 PM
drewbear
hello,
you made no errors in your work. the only problem is that acceleration is the 2nd derivative of position,not the first. you found the velocity equation. so just take the derivative of what you just got and solve
• Feb 25th 2010, 09:25 PM
11rdc11
Quote:

Originally Posted by Zanderist
http://www.mathhelpforum.com/math-he...1&d=1267164367

I know how to begin, take the derivative.

okay that comes out to be around

"5t^4-24t^3"

So I factor out a "t"

"t^3(5t-24)"

then I set equal to "0"

"5t-24=0"

and the answer should be "24/5" but it's wrong. What have I done wrong.

you have to take the second derivative and equal that to 0

\$\displaystyle s''= 20t^3-72t^2\$
• Feb 25th 2010, 09:27 PM
Zanderist
VICTORY!

The second der. is

f ''(x)= "20t^3-72t^2"

factor out a "t"
f ''(x)= "t^2(20t-72)"

The answer is "t = 72/20"