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Re: 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 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.

Lorinda Stinnett


I haven't followed all that has been said about this but here's my take.

When rolling downhill the static friction force does negative work as
far as linear motion is concerned but it does an equal amount of
positive work as a torque acting about the CM of the ball. The net
amount of work done, when the total kinetic is taken into account
(Linear KE of CM + rotational KE about the CM) those two works add to

Going uphill the static friction force actually reverses and the
signs of both works switch so the total still remains zero.

Good Luck,
Herb Schulz