Friday, December 17, 2010

Momentum units

I've been teaching out of the Six Ideas that Shaped Physics for a while now.  One of the things I really like about it is the approach to Newton's laws.  Essentially he starts with conservation of momentum and ends with Newton's third law.  He focuses much of the series on the concept of interactions, making it clear that particles swap momentum during interactions while the particle pairs swap potential energy for kinetic energy.  I've found this fun to teach but I get caught up not having a simple unit for momentum.

I've been heard saying: "When that ball hits the other one, 7 kilogrammeterperseconds are swapped."  I say it really fast because I want the students to recognize that momentum has been transfered from one particle to the other.  I'm writing this post to throw out a suggestion: How about we use the unit "Pom" for momentum?  The word comes from a shortening of what I sometimes call momentum (pomentum) to remind them that the letter we use is p.

Why would this be useful?  Well, then I could say "When that ball hits the other one, 7 poms are transfered"  Rolls off the tongue, doesn't it?  Also, if you've read the Six Ideas books you'll know that he describes force as simply an accounting trick to keep track of a continuous transfer of poms.  That's why the units of force is poms per second (or, if you insist, the awkward "Newton").  If you want to know if something's going to hurt, you just have to see what the pom rate would be.

I've seen how students relate everything to Newtons, or force.  I think, though, and here I think Thomas Moore would agree, it can be very satisfying to try to find how the momentum is being transfered in all the various interactions involved in a physics event.  What do you say?


  1. Yes—This is great. and 1 pom could easily be called a "parcel of momentum" to help cement the visualization that particles are transferring bits of momentum via impulses.

  2. parcel of momentum it is (until a punnier commenter comes up with something better)
    Honorable mention:
    pieces of mud
    Pow! Ouch! Man!

  3. Thought you might enjoy this: an elementary school teacher who may be a kindred spirit.