# Jerk

*This article is about the physics concept of jerk. For other terms of jerk, see Jerk (disambiguation)*

In physics, **jerk** (in British English, **jolt**), also called **surge**, is the derivative of acceleration with respect to time (or the third derivative of displacement). **Yank** is mass times jerk, or equivalently, the derivative of force with respect to time. Jerk is a vector, and there is no generally used term to describe its scalar value.

The units of jerk are metres per second cubed (m/s^{3}). There is no universal agreement on the symbol for jerk, but *j* is commonly used.

Jerk is used at times in engineering, especially when building roller coasters. Some precision or fragile objects—such as passengers, who need time to sense stress changes and adjust their muscle tension, or suffer *e.g.* whiplash—can be safely subjected not only to a maximum acceleration, but also to a maximum jerk. Jerk may be considered when the excitation of vibrations is a concern. A device which measures jerk is called a "jerkmeter."

Jerk is also important to consider in manufacturing processes. Rapid changes in acceleration of a cutting tool, for example going from zero to 100 percent instantaneously, result in theoretically infinite jerk. This can lead to premature tool wear and result in uneven lines of a cut. This is why modern motion controllers include such features as jerk limitation.

Higher derivatives of displacement are rarely necessary, and hence lack agreed names. The fourth derivative of position was considered in development of the Hubble Space Telescope's pointing control system, and called *jounce*. Many other suggestions have been made, such as *jilt*, *jouse*, *jolt* and *delta jerk*. As more distinct terms that start with letters other than "j", *snap*, *crackle* and *pop* have been proposed for the 4th, 5th, and 6th derivatives of displacement, respectively, with some nonzero positive value for tongue-in-cheek.

## External links

- What is the term used for the third derivative of position?, description of jerk in the Usenet Physics FAQ.