Re: [Phys-L] treating force as a vector ... consistently

[Jeff Bigler <jcb@alum.mit.edu>]

On 8/28/2016 1:24 PM, Scott Orshan wrote:
> As a preface, I'm not big on using momentum as a beginner concept. I
> can't see or feel momentum, and it changes with the frame of reference.

Agreed, and I also teach forces and energy before momentum.  However, I
do have a fun activity for students to feel changes in momentum.  I made
a cart out of a 2m x 1m platform and four low-friction, fixed
(non-swiveling) wheels.  I use it to have students experience various
change-in-momentum scenarios.  One of the most instructive is to have
one student riding the cart at a velocity of around 2 m/s and have a
second student jump onto the cart from the side (i.e., with zero
momentum in the direction the cart is traveling).  The instant the
student lands, the cart's velocity changes abruptly and dramatically,
much to the surprise of the rider.  One year, I had a student who had a
high hairline and wore a hairpiece in the front so she would have more
natural-looking bangs.  She was the rider, and when the second (much
heavier) student jumped on, the impulse to the cart was forceful enough
to make the hairpiece fly out of her hair.

Jeff Bigler
Lynn English HS; Lynn, MA