Re: Newton's second law

["Kilmer, Skip" <kilmers@GREENHILL.ORG>]

Wouldn't the rails need to push on the car in the direction the sand is thrown? That would be an outside force.
skip

-----Original Message-----
From: Justin Parke [mailto:FIZIX29@AOL.COM]
Sent: Thursday, November 06, 2003 13:51
To: PHYS-L@lists.nau.edu
Subject: Re: Newton's second law


I wrote:
> > Newton' second law:
> >
> > F = dp/dt= mdv/dt + vdm/dt
> >
> > In the third term, what is v?
> >
> >
> > Justin Parke
> > Oakland Mills High School
> > Columbia, MD

Dan replied
> F = dp/dt = d(mv)/dt = m dv/dt + v dm/dt
>
> This is the chain rule applied to momentum p = mv.
> v is velocity throughout
>
> regards,
>
> Dan M

Consider a flatcar on a train track rolling at speed v without dissipative forces and loaded with sand.  As the car rolls sand is shoveled off to the side at a rate of c, thrown perpendicular to the direction of motion.  Will the speed of the car change as a result of the decrease in mass?

I suspect the answer is no because the force exerted on the sand, and hence the force exerted *by* the sand on the car has no component along the direction of motion.

F = mdv/dt + vdm/dt = m (0) + v (-c)?   This suggests that an outside force is necessary to maintain a constant speed.  What is wrong with my reasoning?

Justin Parke
Oakland Mills High School
Columbia, MD