Re: [Phys-l] circular motion FBD

["Bob Sciamanda" <treborsci@verizon.net>]

Hi John,
I am envisioning that the walker lifts and extends a foot, and then lowers 
it to the ground with a backward swing, so as to push off from the ground. 
That’s how he gets the ground to push him forward. If he doesn’t exert this 
backward foot motion, then you are correct – his forward motion would just 
continue – but instead of pivoting on a ground connection his foot would 
simply slide forward.

From: John Mallinckrodt
Sent: Tuesday, November 01, 2011 12:31 PM
To: Forum for Physics Educators
Subject: Re: [Phys-l] circular motion FBD
Bob,  I don't understand why you think that stepping on a banana peel with 
your forward foot while walking would cause that foot to slip backward.  I'm 
not saying it doesn't--I haven't done the experiment and don't intend 
to!--but it seems all but obvious to my intuition that the foot would slip 
forward.

On the other hand, I *can* imagine it happening to a sprinter who is both 
subject to substantially more air resistance (resulting in a greater 
requirement for a net rearward push) AND is moving each foot into "rearward 
push mode" more quickly (reducing the time allowed for a *forward* slip to 
have any consequences.)

John Mallinckrodt
Cal Poly Pomona

On Nov 1, 2011, at 9:03 AM, Bob Sciamanda wrote:

> I think one must consider the common case in which the walking individual
> already has a forward momentum.  When he then steps on a "frictionless"
> banana peel his forward foot will indeed slip backwards, but his C.M will
> continue forward - with a resulting rotation.


> -----Original Message----- 
> From: John Mallinckrodt
> Sent: Tuesday, November 01, 2011 11:46 AM
> To: Forum for Physics Educators
> Subject: Re: [Phys-l] circular motion FBD
>
> Anthony Lapinski wrote:
>
> > I think the banana peel reduces the friction coefficient, and thus the
> > static friction force is reduced. Your push backward exceeds this, so you
> > slip backward.
>
> As Robert Cohen wrote, this isn't what most folks, especially cartoonists,
> think happen and I'm inclined to agree with them
>
>  http://bit.ly/vQ5mAP
>
> John Mallinckrodt
> Cal Poly Pomona
>
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Bob Sciamanda
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