Originally Posted by barit1
… if you hit a slick spot so that acceleration suddenly goes to zero, then that's "positive jerk". Where you were against the tight shoulder straps, your torso now bounces back against the seat. This gives the PERCEPTION of positive acceleration.
I understand the physics and the derivations involved, and, of course you are correct. Additionally, as it is true in one direction, it is just as true in the other direction. For example: accelerating in an automobile … release the accelerator, engage the clutch and shift, release the clutch, re-engage the accelerator – and so on through the number of forward gears in the example automobile. The occupants in that automobile would be subjected to the same (although opposite) accelerations on their body (and sometimes find themselves bouncing their foreheads off the dash board). However, I have yet to hear of an occupant in an automobile indicating that during this change in the acceleration rate (during the gear change) they felt like they were
decelerating. By the application of the same, legitimate rationale, they would have been subjected to the identical (although, again, opposite) force changes. Might I suggest that it could be merely a recognition factor on the part of the individuals involved? In one case, knowing what to expect and recognizing it when it occurs – and the other case(es?), not knowing what to expect and being surprised when it occurs?