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]

Asymmetrical Newton's Cradle

We spent some time in my high school class today examining the behavior
of the Newton's Cradle (the good ol' clicky-clacky ball thing). After
some thought, my practicum student (pre-service teacher) posed the
following question:
Suppose the fist ball in the cradle had a mass 1.5x while the other
balls in the cradle each had a mass of 1x. What would happen if you
pulled back on that slightly more massive ball and released it?

Cheers,
Matt


"An expert is a man who has made all the mistakes which can be made in a
very narrow field."

- Niels Bohr


-----Original Message-----
From: Bob LaMontagne [mailto:rlamont@POSTOFFICE.PROVIDENCE.EDU]
Sent: Thursday, November 06, 2003 2:08 PM
To: PHYS-L@lists.nau.edu
Subject: Re: Newton's second law

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