Editing Jerk (physics) (section)

==Physiological effects and human perception==
{{see also|g-force#Human tolerance{{!}}Human tolerance of g-force|motion simulator#HumanPhysiologyResponseToMotion{{!}}How human physiology processes and responds to motion}}

Human body position is controlled by balancing the forces of [[Antagonist (muscle)|antagonistic muscles]]. In balancing a given force, such as holding up a weight, the [[postcentral gyrus]] establishes a [[control loop]] to achieve the desired [[mechanical equilibrium|equilibrium]]. If the force changes too quickly, the muscles cannot relax or tense fast enough and overshoot in either direction, causing a temporary loss of control. The reaction time for responding to changes in force depends on physiological limitations and the [[attention]] level of the brain: an ''expected'' change will be stabilized faster than a ''sudden'' decrease or increase of load.

To avoid vehicle passengers losing control over body motion and getting injured, it is necessary to limit the exposure to both the maximum force (acceleration) ''and'' maximum jerk, since time is needed to adjust muscle tension and adapt to even limited stress changes. Sudden changes in acceleration can cause injuries such as [[Whiplash (medicine)|whiplash]].<ref name="thetartan2007"/> Excessive jerk may also result in an uncomfortable ride, even at levels that do not cause injury. Engineers expend considerable design effort minimizing "jerky motion" on [[elevator]]s, [[tram]]s, and other conveyances.

For example, consider the effects of acceleration and jerk when riding in a car:

* Skilled and experienced drivers can accelerate smoothly, but beginners often provide a ''jerky'' ride. When changing gears in a car with a foot-operated clutch, the accelerating force is limited by engine power, but an inexperienced driver can cause severe jerk because of intermittent force closure over the clutch.
* The feeling of being pressed into the seats in a high-powered sports car is due to the acceleration. As the car launches from rest, there is a large positive jerk as its acceleration rapidly increases. After the launch, there is a small, sustained negative jerk as the force of air resistance increases with the car's velocity, gradually decreasing acceleration and reducing the force pressing the passenger into the seat. When the car reaches its top speed, the acceleration has reached 0 and remains constant, after which there is no jerk until the driver decelerates or changes direction.
* When braking suddenly or during collisions, passengers whip forward with an initial acceleration that is larger than during the rest of the braking process because muscle tension regains control of the body quickly after the onset of braking or impact. These effects are not modeled in vehicle testing because [[cadaver]]s and [[Crash test dummy|crash test dummies]] do not have active muscle control.
* To minimize the jerk, curves along roads are designed to be [[Euler spiral|clothoids]] as are railroad curves and [[Vertical loop|roller coaster loops]].