# Homework Help: A quick dirivative Question

1. Apr 28, 2006

### moo5003

Alright, I'm doing a dynamical magnetism problem for my physics class. I've got it almost done except I need to take the derivative of 1/x^3 with respect to time. X in this case is a variable for meters, I want to turn this into an expression with velocity (Velocity is given) but I'm not sure what that is.

d(1/x^3)/dt = ? I'm pretty sure its not just 1/v^3 but that would be nice ^_^. Any help would be appreciated.

Would this just be -4x^-3*v?

Last edited: Apr 28, 2006
2. Apr 28, 2006

### Curious3141

No, you differentiated wrongly. What is $$\frac{d}{dx}(x^{-3})$$ ?

Then $$\frac{d}{dt}(x^{-3}) = \frac{d}{dx}(x^{-3})(\frac{dx}{dt}) = v\frac{d}{dx}(x^{-3})$$ as you correctly surmised.

3. Apr 28, 2006

### moo5003

k, I guess I have a bigger problem then originally thought since X is not given :/.

4. Apr 28, 2006

### Curious3141

What ?

x is a variable denoting displacement right ? And don't mix up the cases - stick to small x.

All I'm saying is you differentiated wrong.

What is $$\frac{d}{dx}(x^n)$$ ? This should be in the textbook. Now plug in n = -3.