Re: That jerk again!

[John Gastineau <gastineau@mindspring.com>]

At 07:39 AM 11/19/97 -0800, Leigh wrote:
> When I asked the question I assumed that many of you had tried
> the same thing at one time or another. I now realize that some
> among you have never driven a car with a standard transmission.
> That may be my communication problem here.

No, I think you were clear. Late last night almost headed out into the cold
with a CBL and accelerometer to play in my truck, but decided against it.
Instead, I taped an accelerometer to a cart, propped up two legs of my desk
to make a ramp (you have to improvise at midnight) and took some data. 

The I let the cart roll up the hill, and back down, and got the expected
nearly constant acceleration. (Nearly, because frictional forces conspire to
make the acceleration on the upward trip slightly larger than during the
downward trip.)

Then, with a little practice (and using anticipation, to considerably better
than 0.25 seconds, thank you) I was able to grab the cart at the top of the
motion, so that the velocity varied linearly down to zero and then stayed
there. The measured acceleration was just what I expected--it was constant,
then jumped to zero. 

I didn't learn anything new from the experiment other than to confirm my
expectations. However, I think that the way we perceive jerk comes into play
in explaining Leigh's experiment on the way to the dentist. 

During a smooth acceleration, we _feel_ a constant force as a pressure on
our bodies. Even flooring the accelerator (on cars that I can afford)
results in a fairly gradual change in acceleration, so we perceive merely a
varying pressure. It's when the pressure changes very rapidly (such as
popping the clutch) that we feel a jerky motion. That is, the rate of change
of acceleration is unacceptably high--large jerk, and a sudden pressure
change, perceived (literally) as a kick in  the seat!

In rolling the car up a hill, it matters not when you jam on the brakes (at
the top or before); you are introducing a sudden change in acceleration, a
jerk, and we feel that pressure change. It is simply surprising to have it
happen at zero velocity. The value of the velocity at a given instant has
nothing to do with its first or second derivative. 

John M. has described the half-roll experiment that we all do, manual
transmission or not, in feathering the brakes so there is no jerk when we
come to a stop at the stop sign. Remember the first time you stopped a car
by just pressing on the brake with constant pressure? You get a big lurch
when the car actually stops if you don't reduce the braking force, and so
smoothly vary the acceleration down to zero. 

JEG

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John E. Gastineau      gastineau@mindspring.com               KC8IEW
900 B Ridgeway Ave.    http://gastineau.home.mindspring.com        
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