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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

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
answer is achieved.
Oren Quist, SDSU

-----Original Message-----
From: lorinda.stinnett [mailto:stinn001@BAMA.UA.EDU]
Sent: Tuesday, June 26, 2001 3:56 PM
Subject: Conservation of ME and nonconservative forces


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

A ball rolls down an inclined plane without slipping. Given the
from which it starts, find the speed of the ball at the bottom of the
So we look at conservation of ME.

But in order to use cons. of ME, we need to be certain that there is
work done by nonconservative forces.

Forces on ball:
1. Gravity - conservative - OK
2. Normal - does no work on object - perpendicular to direction of
- 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.

Lorinda Stinnett

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