# Difference between jerk in accerlation and deceleration?

1. Apr 11, 2013

### Seminole Boy

Yes, I'm back to this Einstein jerk, but it's in an entirely different context. Please, mentors, do not close this until my problem is resolved.

Okay, when one accelerates, one feels the Einstein jerk (discussed midway through his book). When one decelerates, one feels the Einstein "jerk".

If space has no bounds, this means there is no real direction. The jerks are experienced "backwards" and "forwards", but that's essentially the same thing or the same "direction." Going "north" through spacetime is the same as going south, west, or east through spacetime. And the jerk the body experiences is the same whether it's done by acceleration or deceleration.

Okay. So how is acceleration any different from deceleration?

2. Apr 11, 2013

### Passionflower

Deceleration is simply acceleration in the opposite direction, so it is not different at all.

3. Apr 11, 2013

### Seminole Boy

Okay. Thank you. That's what I wasn't understanding. Mentors, feel free to close this. I should have just emailed you or the Great Peter Donis.

4. Apr 11, 2013

### Passionflower

One more note: I notice you use the term jerk which has a specific meaning in acceleration.

While acceleration is the second derivative of position, jerk is the third derivative of position. Velocity is the first derivative of position. All with respect to time.

Interestingly higher than 3 derivatives start to give 'problems' in relativity, if I remember correctly things no longer commute in higher derivatives with a result that there is no single one answer.

5. Apr 11, 2013

### Seminole Boy

Passionflower, now you're confusing me. You're saying there is some kind of position hierarchy?

6. Apr 11, 2013

### Passionflower

When something starts to move there are actually an infinite number of higher derivatives, but usually scientists do not worry too much about higher derivatives than 2.

7. Apr 11, 2013

### Staff: Mentor

I'd be interested in some examples/references.... Not disagreeing, but curious.

8. Apr 11, 2013

### Passionflower

I looked around if I could find something.

See for instance chapter 2.1 and 2.2 in "Relativistic Kinematics and Stationary Motions" where jerk and snap are defined. http://arxiv.org/abs/0902.4243

In chapter 7 it states:

In the instantaneous rest-frame one can define the proper jerk j as in non-relativistic mechanics, but the D-vector jerk should not be defined simply as the proper-time derivative of the D-acceleration because (i) it is not orthogonal to the D-velocity U, and hence may be timelike, and (ii) it does not vanish for worldlines of constant|A|.
...
In contrast to the notions of proper acceleration and proper jerk, there is an ambiguity in the definition of proper snap, arising from the fact that the triple derivative with respect to coordinate time t does not coincide in the instantaneous rest-frame with the triple derivative with respect to proper time, whereas there is a coincidence for single and double derivatives