Re: Conservation of ME and nonconservative forces

["QUIST, OREN" <OREN_QUIST@SDSTATE.EDU>]

Yes there is static friction.  It is static because at the instantaneous
point of contact there is no motion of the surface of the ball (disk) with
respect to the plane.  The ball is rotating exactly fast enough so that the
(instantaneous) bottom of the ball is moving "backwards" to be at rest with
the plane.

The point of contact is moving, but at the point of contact the surface of
the ball is not moving with respect to the plane.

Further, if you use the coefficient of kinetic friction, then the force of
friction has to be mu sub k times the normal force.  If you use static
friction, then the force of friction is <= mu sub s times the normal force.
You must use the "less than" part of this equation, as the force of friction
cannot be larger than that necessary to produce a torque to produce the
angular acceleration resulting in the correct angular speed at any instant.
At the maximum angle prior to slipping you will end up using the equals
sign, but at any angle below this, you find the force of friction from the
necessary torque condition.

-----Original Message-----
From: Paul O. Johnson [mailto:hodgepoj@YAHOO.COM]
Sent: Wednesday, June 27, 2001 10:12 AM
To: PHYS-L@lists.nau.edu
Subject: Re: Conservation of ME and nonconservative forces


There is indeed no slipping, Oren, so there is no KINETIC friction. But
it seems to me that there must be STATIC friction to cause rolling. It
also seems to me that the instantaneous point of contact is indeed
moving down the plane, opposite to the static frictional force.
Therefore, the frictional force does negative work, converting
potential energy to thermal energy.

Paul O. Johnson
The Science Place
Dallas, Texas

--- "QUIST, OREN" <OREN_QUIST@SDSTATE.EDU> wrote:
> Friction does no work in this case because there is no slipping.
> That is,
> there is a force but no displacement of the contact point.
>
> The problem is easily done using torques and Newton's Laws and the
> same
> answer is achieved.
> Oren Quist, SDSU
>
> -----Original Message-----
> From: lorinda.stinnett [mailto:stinn001@BAMA.UA.EDU]
> Sent: Tuesday, June 26, 2001 3:56 PM
> To: PHYS-L@lists.nau.edu
> Subject: Conservation of ME and nonconservative forces
>
>
> Hello,
>
> I am teaching the first semester of the introductory calc based
> Physics -
> first time.
>
> I am in need of a good freshman level explanation to the following
> problem:
>
> A ball rolls down an inclined plane without slipping.  Given the
> height
> from which it starts, find the speed of the ball at the bottom of the
> incline.
> So we look at conservation of ME.
>
> But in order to use cons. of ME, we need to be certain that there is
> no
> work done by nonconservative forces.
>
> Forces on ball:
> 1.  Gravity - conservative - OK
> 2.  Normal - does no work on object - perpendicular to direction of
> motion
> - OK
> 3.  Static friction at the point of contact between ball and surface.
> ??
>
> I am believe I understand that the static friction does no work, I am
> looking for an explanation my students would understand.
>
> Thanks,
> Lorinda Stinnett


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