Re: Jerk

["Dewey Dykstra, Jr." <dykstrad@varney.idbsu.edu>]

> Someone said jerk (the third time derivative of displacement) is a
> kinematic, not a dynamic quantity. I maintain that is not a useful
> point of view. Jerk has important dynamic consequences. Anyone who
> thinks otherwise must have failed to notice what happens when one
> is standing up in the aisle of a bus and the driver leaves the bus
> stop with a jackrabbit start. In this case the kinematic jerk is
> the driver, of course, but the change in the direction of the frame
> force felt by the standee is dramatic and is the consequence of the
> dynamic jerk.
>
> Leigh

Now I'm not sure if the meaning here would be changed if a ";^)" followed
it or not.  ;^)  Is this a serious response?

If "jerk" is defined as the 3rd derivative of the displacement with respect
to time, then it is officially considered a kinematic entities just as
velocity and acceleration are; i.e. descriptions of the motion.  As I
understand it the adjective "dynamic" is used to describe entities which
are deemed causes or modifiers of the motion.  I believe that this
distinction is preserved in all the texts that I know of.  (Or are ALL the
authors wrong in this respect, too?)

Hence, I believe that officially "jerk" IS a consequence and not a cause.

To remain standing on the bus either you or some part of the bus or another
passenger must apply the forces to your body necessary to match (and make
up for any lags) the acceleration and jerk of the bus; i.e. your kinematics
must sufficiently match the kinematics of the bus.  Dynamic entities,
forces in particular resulting in a net force in particular, are, in the
Newtonian system, considered the cause.

It is also the case that probably 99+% of the ridership of buses manages to
learn to deal with this experience and never have any thought of a
distinction between "kinematic" and "dynamic".

The distinction is not whether or not one is going to feel something or
respond to something, but whether an entity is the description of the
motion (displacement, velocity, acceleration, jerk) or not such as a cause
or modifier of the motion (force, inertia, etc.)

Sheesh, Dykstra giving an official definition, what's the world coming to...?!
;^)

Dewey

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Dewey I. Dykstra, Jr.                   Phone: (208)385-3105
Professor of Physics                    Dept:  (208)385-3775
Department of Physics/SN318             Fax: (208)385-4330
Boise State University                  dykstrad@varney.idbsu.edu
1910 University Drive                   Boise Highlanders
Boise, ID 83725-1570                       novice piper
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