Re: Can a system's mass vary?

[Robert Cohen <Robert.Cohen@PO-BOX.ESU.EDU>]

I asked:

> Does it matter what the speed of your inertial reference 
> frame is?  In other words, does it still work if you observe 
> the grain elevator/hopper car system from a moving train?

to which Mark Shapiro replied:

> Not if you are careful to properly identify v as the relative 
> velocity of the car and the grain elevator.

What is it about applying dp/dt with the mass varying
that requires v to be the relative velocity?

It seems to me that you aren't really using dp/dt with
the mass varying.  Rather, you are applying Newton's
second law twice - once to the grain and once to the
hopper - with the provision that the grain's initial
momentum is zero.  The end result:

0 = m dv/dt + v dm/dt

"looks" like you've applied dp/dt with the mass varying
but is that really what you've done?

____________________________________________________
Robert Cohen; 570-422-3428; www.esu.edu/~bbq
East Stroudsburg University; E. Stroudsburg, PA 18301