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# pity the driving instructor

01/23/2009 1:01 PM

Your sister comes home from a driving lesson and says"Hey,my driving instructor said we want to brake so as to remove from the car at a constant rate,and that's going to give
the smoothest results.I think he's a twerp-that's not going to work very well."Your first response is,"Is that what you told him?!When she reassures you she's not going to be kicked out of driving class,you start thinking about it.Is she right?

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#1

### Re: pity the driving instructor

01/23/2009 1:54 PM

Is this a physics 101 homework question?

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#2

### Re: pity the driving instructor

01/23/2009 2:57 PM

the wording sounds like homework to me.

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#3

### Re: pity the driving instructor

01/23/2009 3:45 PM

Is she right about the guy being a twerp?

Are you asking (implication is sorta embedded in your post) whether or not the instructor is right about braking such that (here's where I have to assume that you meant to say somethig about speed) the speed reduces at a constant rate (e. g. constant negative acceleration)? If so, the answer is "no," that's not the "smoothest" way to brake. The smoothest way to brake is to first negatively accelerate at an increasing rate until half the speed is reached, at which point one switches to negatively accelerate at a decreasing rate (looks like the first 180o of a cosine curve). Such a speed curve has no discontinuities.

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#4

### Re: pity the driving instructor

01/23/2009 4:14 PM

or...

x = displacement

v = velocity

a = acceleration

t = time

v = dX / dt

a = dv/dt

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#5

### Re: pity the driving instructor

01/23/2009 6:07 PM

I think the driving instructor is very smart, though perhaps not entirely correct when the laws of physics are imposed.

He's talking, presumably, to a teenager, who could care less about the math, but he may connect with her if he says it makes it smooth.

Keep it simple.

LL

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#6

### Re: pity the driving instructor

01/24/2009 12:51 AM

In motion control a S curve acceleration / deceleration beats a ramped (constant) movement for smoothness.

But a leg is not CNC controlled so I would imagine exercising braking for it to feel smooth would produce a S curved stop. Just beware of running out of road.

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#7

### Re: pity the driving instructor

01/24/2009 11:00 AM

a leg is not CNC controlled

... or at least works with fuzzy logic...

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#8

### Re: pity the driving instructor

01/24/2009 3:34 PM

> ... or at least works with fuzzy logic...

Being that it is his sister, and she is of legal driving age, she is probably shaving her legs... so it shouldn't be too fuzzy... . Is there such a thing as smooth logic? Smooth-or-slightly-prickly-if-it's-been-a-few-days logic?

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#9

### Re: pity the driving instructor

01/26/2009 10:34 AM

Just taking your foot off the accelerator will lower your center of gravity especially when going around a curve, whereas stepping on the brake raises your center of gravity.

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