# Rolling on flat floor, rolling down an inclined plane?

1. Jan 13, 2014

### applestrudle

When rolling on a flat floor when the velocity at the circumference equals the velocity at the centre of mass the friction stops acting right? SO why does't that happen when it is rolling down an inclined plane? If it is rolling shouldn't the point in contact with the inclined plane be stationary relative to the inclined plane (therefore no friction just like on the flat floor)??

Please explain this to me

2. Jan 13, 2014

### tiny-tim

hi applestrudle!
when rolling at constant speed, there is no angular acceleration (and, incidentally, no horizontal linear acceleration)

so there is no torque

so no friction​

(when rolling at constant speed against air resistance and/or against an external frictional torque at the axle, the total torque must be zero, so the friction with the road must be non-zero so as to balance out the air resistance or axle friction:

so you see, the stationaryness of the point of contact is irrelevant)
when rolling downhill, there is angular acceleration

so there is torque (and the torque of both the weight and the normal force about the centre of mass is zero) …

so there is friction