Re: [Phys-L] ? FCI --> momentum flow

[Jeffrey Schnick <JSchnick@Anselm.Edu>]

> -----Original Message-----
> From: Phys-l [mailto:phys-l-bounces@phys-l.org] On Behalf Of John Denker
> Sent: Tuesday, October 22, 2013 5:31 PM
> To: Phys-L@Phys-L.org
> Subject: Re: [Phys-L] ? FCI --> momentum flow
>
>
> Allowed possibilities include:
>  a) In the lab frame, the book is not accelerating.  Its
>   velocity is not changing and its momentum is not changing.
>  b) In a local freely-falling frame, there is no gravitational
>   acceleration.  The book is steadily accumulating upward
>   momentum due to the unbalanced forces, namely the forces
>   associated with contact with the table, the other books,
>   et cetera.  The momentum is changing (in this reference
>   frame).  The velocity is changing (relative to this
>   reference frame).
It's b) that I had in mind.  Harry is viewing the situation from an inertial reference frame that was commoving with the lab frame at time 0.  He's the one that tells us that there is no gravitational force.  He's the one that tells us that the reason we don't notice the book gaining momentum and velocity is because we are riding along right beside the book--our velocity and momentum are changing too.  He claims that there is no gravitational force (and hence we shouldn't concern ourselves with upward flow of downward momentum) and advises us that if we want to determine whether or not some apparent force is an actual force in accord with Newton's third law, we have to look at the situation from an inertial reference frame.