[Physics FAQ] - [Copyright]

Updated by Stephanie Gragert, 1998.
Original by Philip Gibbs, 1996.

What is the term used for the third derivative of position?

The first derivative of position (symbol x) with respect to time is velocity (symbol v), and the second derivative is acceleration (symbol a).  Less well known is that the third derivative, i.e. the rate of increase of acceleration, is technically known as jerk 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 is 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 technical definition so well.  For example, impulse is more commonly used in physics to mean the increase of momentum imparted by a force of limited duration [Belanger 1847], and surge is used by electricians to mean something like an increasing current or voltage.  The terms jerk and jolt are therefore preferred for rate of increase of acceleration.  Jerk appears to be the more common of the two.  It is also recognised in international standards:

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 both acceleration and jerk 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, the rate of increase of jerk.  The term jounce has been used, but has the drawback of using the same initial letter as jerk.  Another less serious suggestion is snap (symbol s), crackle (symbol c) and pop (symbol p) for the 4th, 5th and 6th derivatives respectively.  Higher derivatives do not yet have names because they seldom appear in physics.

Since force (F = ma) is the rate of increase of momentum (p, symbol clashes with pop), perhaps we might want to name higher derivatives of force.  So far, yank (symbol Y) has been suggested for rate of increase of force, tug (symbol T) for rate of increase of yank, snatch (symbol S) for rate of increase of tug, and shake (symbol Sh) for rate of increase 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!!