# Chain rule and dot product

1. Apr 13, 2012

### MeMoses

1. The problem statement, all variables and given/known data
I have vector R. I need to show the R dot dR/dt = 0 => 1/2 d/dt[R dot R]

2. Relevant equations

3. The attempt at a solution
I guess I've never really applied the chain rule to dot products and its throwing me off. How does one go from R.dR/dt=0 to 1/2 d/dt[R.R] = 0. I have no idea to be honest. I'm not sure which direction to take it, all I know is it uses the chain rule. Sorry I haven't done much with this. I hope you can help. Thanks

2. Apr 13, 2012

### Curious3141

Actually it uses the product rule, which applies to vector dot products too. See: http://en.wikipedia.org/wiki/Product_rule#For_vector_functions

Try applying that on the second part, i.e. what is d/dt (R.R)?