Re: Newton's second law

[Bob LaMontagne <rlamont@POSTOFFICE.PROVIDENCE.EDU>]

If you are shoveling perpendicular to the motion of the flatcar, you are
applying a force perpendicular to the track. That is balanced by a force
from the track that keeps the overall motion straight ahead. The v in
this case is the sideways speed of the sand, not the speed of the
flatcar down the track - which remains constant because there is no
force in this direction.

Bob at PC

Justin Parke wrote:
> 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