Re: [Phys-l] geometry of spacetime (was: relativisitic mass ...)

[John Mallinckrodt <ajm@csupomona.edu>]

> The problem I have with relativistic mass is that conceptual level texts use
> it to explain why it is more difficult to accelerate an object when it is
> traveling at high speeds. Unfortunately, they are implicitly using an F=ma
> approach - which means that they are actually talking about the old
> "longitudinal" mass (a 3/2 power) - not the relativistic mass (a 1/2 power
> of 1-v^2/c^2). Invariant mass and the use of F=dp/dt avoids this.

Not that I disagree with the thrust of these comments, but as a side 
note I would offer my opinion that we also needlessly confuse matters 
by saying things like "it is more difficult to accelerate an object 
when it is traveling at high speeds" for the simple reason that 
objects do not *intrinsically* "travel at" any--let alone 
high--"speed" and, therefore, do not become "more difficult to 
accelerate."

It should be very well understood that an object can, in principle, 
experience an arbitrarily large, constant acceleration for an 
arbitrarily large amount of time.  It can do so by firing its 
thrusters at a constant level of thrust.

The fact that that object *appears* to another observer to 
asymptotically approach the speed of light is the *observer's* 
problem!

--
John "Slo" Mallinckrodt

Professor of Physics, Cal Poly Pomona
<http://www.csupomona.edu/~ajm>

and

Lead Guitarist, Out-Laws of Physics
<http://www.csupomona.edu/~hsleff/OoPs.html>