# Static or Kinetic Friction?

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#### chandler742

##### Senior Member
7+ Year Member
15+ Year Member

As many of you guys know, friction can be an unwanted byproduct of many different processes.

However, friction does play a vital role in the movement of tires on the road.

So, Which friction is important for a tire that is in rotational motion on the road(assume no slipping), static friction or kinetic friction?

#### limit

##### Molesting my inner-child
10+ Year Member
15+ Year Member
The instantaneous velocity of a rolling object's point of contact is zero (as the tire is rolling, there is a velocity vector pointing forward, and this is cancelled out by a vector pointing backwards at the point where the tire is touching the ground), so it being instantaneously at rest, we must use static friction to evaluate such problems of rotational motion.

This came up once upon a time, when I was determining the rate of loss of tire molecules on a car tire.

#### freakazoid

##### Guy Friend Extraordinaire
7+ Year Member
15+ Year Member
The power of anti-lock brakes . . .

#### chandler742

##### Senior Member
7+ Year Member
15+ Year Member
you are right. Good job.

Paradoxically, even though the tire is moving the answer is static friction because the condition given was that the tire "doesn't" slip off of the road during rotation.