# Homework Help: Calculus quick question

1. May 8, 2007

### Davio

1. The problem statement, all variables and given/known data
y = A cos (kx-wt)

differentiate twice is -A^2k^2 cos (kx-wt)
Why?

2. Relevant equations
n/a

3. The attempt at a solution
-w^2 A cos (kx-wt)

2. May 8, 2007

### Hootenanny

Staff Emeritus
Apply the chain rule.

3. May 8, 2007

### arunbg

Yes, you should use chain rule, but more importantly,what are you differentiating the function with respect to ? The answer the text gave has a quantity A^2, which I believe can't be obtained by differentiating the eqn, wrt any given variable in the function. Please check if you made an errors in copying the answer.

4. May 8, 2007

### Hootenanny

Staff Emeritus
Good catch arunbg, didn't notice that myself

5. May 8, 2007

### arunbg

Well, on second thought , you can get the answer if you derive the function with respect to x/A treating A as constant (using good old chain rule), but then that would be calculus, not physics

Oh, and hoot congrats for goin gold, and keep up the good work.

6. May 8, 2007

### Hootenanny

Staff Emeritus
Thanks arunbg , much appreciated. I've not 'seen' you much on the forums lately, hope everything is good at your end.