# Homework Help: Help mechanics

1. Feb 14, 2010

### nna

I hope somebody can help me.. this is the problem i have to proof that if A is a constant vector then I can write its derivate as dA/dt = \omega x A.. where \omega is a vector, and the "x" is the cross product

2. Feb 14, 2010

### tiny-tim

Welcome to PF!

Hi nna! Welcome to PF!

(have an omega: ω )
(You mean "if A is constant in magnitude".)

Hint: if the magnitude is constant, then so is the magnitude squared, which is A.A.

3. Feb 14, 2010

### nna

Sorry :( but I don't understand how that helps...

4. Feb 14, 2010

### tiny-tim

Differentiate it …

what do you get?

5. Feb 14, 2010

### nna

ok ok thank you so much!!! it really helps... jaja it was very easy sorry