What is the term used for the third derivative of position?
It is well known that the first derivative of position
(symbol x) with respect to time is velocity
(symbol v) and the second is acceleration
(symbol a). It is a little less well known that
the third derivative, i.e. the rate of change of acceleration, is
technically known as jerk (symbol j).
Jerk is a vector but may also be used loosely as a scalar quantity
because there is not a separate term for the magnitude of jerk
analogous to speed for magnitude of velocity.
In the UK jolt has sometimes been used instead of jerk and
may be equally acceptable.
Many other terms have appeared in individual cases for the third
derivative, including pulse, impulse, bounce, surge, shock and super
acceleration. These are generally less appropriate than jerk
and jolt, either because they are used in engineering to mean other
things or because the common English use of the word does not fit
the meaning so well. For example impulse is more commonly used
in physics to mean a change of momentum imparted by a force of
limited duration [Belanger 1847] and surge is used by electricians to mean something
like rate of change of current or voltage. The terms jerk and jolt are therefore
preferred for rate of change of acceleration. Jerk appears to be
the more common of the two. It is also recognised in international
In ISO 2041 (1990), Vibration and shock - Vocabulary, page 2:
"1.5 jerk: A vector that specifies
the time-derivative of acceleration."
Note that the symbol j for jerk is not in the standard
and is probably only one of many symbols used.
As its name suggests, jerk is important when evaluating
the destructive effect of motion on a mechanism or the
discomfort caused to passengers in a vehicle. The movement of
delicate instruments needs to be kept within specified limits of
jerk as well as acceleration to avoid damage. When designing
a train the engineers will typically be required to keep the jerk
less than 2 metres per second cubed for passenger comfort.
In the aerospace industry they even have such a thing as a
jerkmeter; an instrument for measuring jerk.
In the case of the Hubble space telescope, the engineers are
said to have even gone as far as specifying limits on the
magnitude of the fourth derivative. There is no universally
accepted name for the fourth derivative, i.e. the rate of change
of jerk, The term jounce has been used but
it has the drawback of using the same initial letter as jerk
so it is not clear which symbol to use. Another
less serious suggestion is snap (symbol s),
crackle (symbol c)
and pop (symbol p) for the 4th, 5th and
6th derivatives respectively. Dork has also been
suggested for the sixth derivative. Although the reasons given
were less than entirely sincere, dork does have an appealing ring to
it. Higher derivatives do not yet have names because they do not
come up very often.
Since force (F = ma) is rate of change of
momentum (p, symbol clashes with pop)
it seems necessary to find terms for higher derivatives of force too. So
far yank (symbol Y) has been suggested for rate
of change of force, tug (symbol T)
for rate of change of yank, snatch (symbol S)
for rate of change of tug and shake (symbol Sh)
for rate of change of snatch. Needless to say, none of these are in any
kind of standards, yet. We just made them up on usenet.
Now class, repeat after me...
"Momentum equals mass times velocity!
Force equals mass times acceleration!
Yank equals mass times jerk!
Tug equals mass times snap!
Snatch equals mass times crackle!
Shake equals mass times pop!!