constant speed rolling motion

[flasherffff]

hi ,this is my first post (and its not HW)
i currently study dynamics of rigid bodies
using (Hibbeler)

and theres one thing about this type of motion (rolling) that just seems to elude me

for any planar motion we have 3 equations
\Sigma F_{{x}}={\it ma}_{{g_{{x}}}}
\Sigma F_{{y}}={\it ma}_{{g_{{y}}}}
\Sigma M_{{g}}=i_{{g}}\alpha

if i have a wheel rolling on a surface with constant velocity ,say a car wheel not connected to the engine

by kinematics a_{{g}}=\alpha\,r then \alpha=0

then there is a force P acting on the mass center G
then by the first equation if ag=0
then the friction force f must be equel and opposite of p and acting at the contact point A

but then by eq3 the moment about G causes an increase in \alpha
contradicting the kinematics equation

http://img5.imageshack.us/img5/4869/scanpic0001m.jpg (http://img5.imageshack.us/i/scanpic0001m.jpg/)

now ,obviously i got something wrong but what

[Doc Al]

I don't understand. You say constant velocity, yet you have some force P acting? Where does that come from?

[flasherffff]

i assume there is some rolling friction beforehand

[K^2]

If there is a force acting on the center of the wheel, one or both of the two statements is true.

1) The car is accelerating/decelerating.
2) The car is applying torque to the wheel via brakes/engine.

[flasherffff]

but if i dont apply a force
the rolling motion would stop due to friction

[Doc Al]

but if i dont apply a force
the rolling motion would stop due to friction
What friction? Are you considering rolling friction? First get the ideal case straight--perfectly rigid wheel with no friction to worry about.

[flasherffff]

why a perfectly rigid wheel would experience no friction
i mean static friction at the point of contact

[Doc Al]

why a perfectly rigid wheel would experience no friction
i mean static friction
Unless the wheel/surface deform--the source of dissipative rolling friction which stops real wheels from rolling indefinitely--there's no friction to worry about. No static friction is required to maintain the motion--it's constant velocity. The static friction is zero if the wheel isn't accelerating.

[flasherffff]

thanks ,i understand what i got wrong
i assumed there is a constant friction resisting to rolling just like the kinetic friction for sliding

[K^2]

You start with alpha=0 which means no acceleration, which is true only if there is no friction or if there is a torque applied to counter friction.