Field of Science

Working up an appetite for momentum

This week we're "down the shore" as they say around here. We go to the same place each year and my kids have their traditional activities. For example, on the hottest day of the week we're there, we should rent a surrey (a pedicab with two seats and four sets of pedals -- this is not a light weight vehicle) and pedal it up and down the boardwalk for an hour, dodging pedestrians and other cyclists, until the parents (who provide most of the kinetic energy in this event) are soaked in sweat. The ride ends with a short ramp off the boardwalk, which this year Crash Kid was certain he could negotiate without parental assistance. Given his recent history with wheels, his parents were a bit less confident. The conversation quickly turned to momentum, and my spouse noted we had a lot of "m" in the buggy. My youngest thought this might stand for momentum. "Nope, that's 'p'!" his mom replied. Sensibly, he wanted to know why that letter!

Good question, and I'm not sure that I have a good answer for him yet. The lore seems to be that Newton used the term "impetus" in the Principia. Impetus is a Latin import, from petere to seek. Interestingly, the Indo-European root of petere is pter which gives us the Greek pteron (wing), and eventually helicopter. Petere is also the root of the English appetite -- which pedaling that surrey certainly worked up.

If anyone has other ideas about why "p" is used for momentum, I'd love to hear them!

1 comment:

  1. After looking though my classical mechanics text, doing some creative Googling, searching various commentaries on Newton, and skimming parts of the Principia in Latin and English, I have this to report:

    Newton used velocitas for velocity, quantitas materiae for mass, quantitas motus for momentum ("quantity of motion"), and impetus for impulse. In any case, the equations are mostly written out in longhand.

    Momentum is a Latin word meaning motion/momentum. I can't seem to find out how this became the name for the "quantity of motion" or why p became the variable. Anyone up for going though the manuscripts of Lagrange and Hamilton?

    (I'd also like to know why q is used for generalized coordinate. Probably symmetry with p, which leads us back to...)


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