Chronology Current Month Current Thread Current Date
[Year List] [Month List (current year)] [Date Index] [Thread Index] [Thread Prev] [Thread Next] [Date Prev] [Date Next]

Re: special relativity: accelerated frames



Regarding Chuck B's observation:
...
If I choose my car to be my RF, and notice that the highway, trees
etc are going past me at a reasonable constant speed of 20 m/s and I
suddenly mash down on the middle pedal of my car.

I notice that the stuff piled on the back seat of my car 'rushes
forward' onto the floor, (presumably, somehow pulled forward by the
outside world) but the things outside my car seem to be indifferent
to my having stopped the world from moving.

Actually, this is not correct. *Both* the stuff on the back seat *and*
the stuff outside the car experience a similar forward (i.e. along the
direction from the back to the front of the car) acceleration when
you "mash down on the middle pedal of" your car. The only difference
in the two cases is that the stuff on the back seat accelerates in this
direction from an initial state rest and ends up going toward the front
of the car, and the outside stuff accelerates in this direction from
an initial state of motion toward the back of the car. When this same
acceleration is applied to the backwards moving external stuff, it slows
down until you succeed in completely stopping the external world.
When you mash on the middle pedal the only things not experiencing this
additional forward acceleration are items rigidly attached to the car
(such as your, hopefully, seatbelt-restrained self), and even then,
the only reason for them not participating in this forward acceleration
is the fact that the restraints exert contact backward forces on these
restrained bodies, which prevent it from happening.

...
(I ALSO have to wonder how my little Saturn is able give the highway
such a tremendous amount of KE and have the brakes absorb it - but
Leigh would just say that I am reifying energy and should stop DOING
that!! :-)

Actually, what's happening in this frame is that objects naturally
have their energy change as this nonconservative g field becomes
explicitly time-dependent (and this gives rise to time-dependent
forces which change the appropriate KEs when objects yield to those
forces). What your brakes are actually doing is dissipating the extra
energy that the car (and stuff attached to it) would have
acquired in response to this time dependent g field, so that the car
always remains at rest in this frame and so it doesn't participate
in the acceleration that the rest of the world does.
........................................................................

BTW, in my last post on this thread I accidentally put one too many
negations in a statement (I used a triple negative) that invalidated
its meaning. In my last paragraph I wrote:

In any event, what this exercise shows if that *if* we could invent a
rocket propulsion technology that would allow accelerations comparable
to 1 g for a few tens of years, then there is no place in the currently
observable universe which would not be unaccessible to human visitation
^^^
over the lifespan of a single human being ....

What I meant to say was "then there is no place in the currently
observable universe which *would* be unaccessible to human visitation..."

David Bowman
David_Bowman@georgetowncollege.edu