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Re: [Phys-l] car skidding and spinning



The problem with " turn in the opposite direction of whichever way you turned to cause the skid" is that the skid may not have been initiated by a turn - a mistimed press on the accelerator or brakes over a surface that is unevenly slick can skid you in a direction you never expected. You have to react by reflex. Your eye sees which way the car is sliding (rotation doesn't really matter) and you steer in that direction. The point is to get the front wheels rolling instead of sliding. When you get good at it you can steer a car around a corner totally in a skid by coordinating your steering and the accelerator. We'll have snow pretty soon - try it in a vacant Walmart parking lot. Just be careful of the lamp posts :-)

Bob at PC

-----Original Message-----
From: phys-l-bounces@carnot.physics.buffalo.edu [mailto:phys-l-
bounces@carnot.physics.buffalo.edu] On Behalf Of Scott Orshan
Sent: Tuesday, December 06, 2011 3:41 PM
To: phys-l@carnot.physics.buffalo.edu
Subject: Re: [Phys-l] car skidding and spinning

I always thought that the advice to "steer in the direction of the skid"
was some of the most useless advice ever given.

What exactly is the direction of the skid? There's a linear direction, and a
rotational direction. If you steer towards the direction of the linear
displacement, you can arrest the skid. If you steer further into the rotational
direction, you won't stop the skid, and you may even increase the rate of
rotation.

The advice should be to turn in the opposite direction of whichever way you
turned to cause the skid. If the skid was caused by braking, where the back
end of the car is coming around, you've got to steer opposite the direction of
rotation.

If you over correct, you'll start an oscillation that will quickly send you out of
control.

Your strategy may also depend on what is in your path. If you can avoid a tree
by skidding even further, then it helps you to do that. If you can collide with a
barrier while going backwards, you may be less injured that if you hit it head
on. If the strategy would send you into oncoming traffic, it's a bad one.

At high speeds, it all happens too fast to do much about it anyway unless
you've practiced it.

Scott


On 12/5/2011 11:04 AM, phys-l-request@carnot.physics.buffalo.edu wrote:
Message: 2
Date: Mon, 5 Dec 2011 04:15:00 +0000
From: "LaMontagne, Bob"<RLAMONT@providence.edu>
Subject: Re: [Phys-l] car skidding and spinning
To: Forum for Physics Educators<phys-l@carnot.physics.buffalo.edu>
Message-ID:

<9DD8E6B7DA56BD4CA13E0C068DA57218432CC965@DAG01.provide
nce.col>
Content-Type: text/plain; charset="us-ascii"

If you turn into the skid you decrease the friction on the front end of the
car because the wheel can roll instead of skid. This decreases the torque
from the front and the car tends to straighten out. This is an old "dirt track"
trick. It also works great on snow covered roads.

Bob at PC

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