Re: Four-Velocity and four-acceleration question.

From: Dirk Van de moortel (dirkvandemoortel_at_ThankS-NO-SperM.hotmail.com)
Date: 02/22/05 

Date: Tue, 22 Feb 2005 09:42:11 GMT

"GR_GR" <nyb@colorado.edu> wrote in message news:cvejhe$d8q$1@peabody.colorado.edu...
> Dave Snead wrote:
> > a . u = du/dtau . u = 1/2 d/dtau (u . u) = 1/2 d/dtau(-1) = 0
>
>
> Your factor of 1/2 makes sense using the argument I gave. However,
> perhaps you can be a bit more clear as to your argument for the factor
> of 1/2.

Standard product rule:
    d/dtau( u.u ) = u . du/dtau + du/dtau . u
                       = du/dtau . u + du/dtau . u
                       = 2 du/dtau . u
Divide by 2
    1/2 d/dtau( u.u ) = du/dtau . u

Dirk Vdm