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]

Re: That jerk again



Martha wrote:
Here is the way I understand "jerk".
Say I am on a commuter train, standing up, as the train comes into the
station. As it slows to a stop, it has an approximately constant
acceleration opposite the direction of motion. Once it comes to a stop,
of course it doesn't reverse direction but it stays stopped. In other
words, when the velocity reaches zero, the acceleration also
decreases to zero. That's the "jerk", and I think it's the same thing
that happens to Leigh's car--the only difference being the source of
. . .
Now why do I FEEL the "jerk"? . . .
--
Martha Takats

The force stopping your body from further motion is not uniformly applied
over your body (as would a gravitational force, which would also give all)
body parts the same acceleration). If you are standing, this force is
applied at your feet. The other body parts are connected to the feet
by the counterpart of the car's suspension system (especially your innards,
"floating" in the abdominal cavity). What you "feel" is this fight
between the inertia of your body parts and the stopping force from your
feet which is transmitted over body linkages. This also happens to Leigh's
body; but there, we (at least I) were discussing the motion of the car body.
Another complication in your "standing case" is that the external force at
your feet produces a torque on your body.

-Bob

Bob Sciamanda sciamanda@edinboro.edu
Dept of Physics sciamanda@worldnet.att.net
Edinboro Univ of PA http://www.edinboro.edu/~sciamanda/home.html