When Does Static Friction Act in the Direction of Motion?

[vivekfan]

Homework Statement

This is a conceptual question. I always thought that friction goes in the direction opposite of the intended motion. But then today my teacher put a ball in his hand and moved it horizontally back and forth. The ball remained stationary. Then he said static friction is moving in the direction of the motion. Then when he was discussing centripetal acceleration, he said that in a situation where you were in a car making a turn, the static friction is the force keeping you going in a circle. So the static friction is in the direction of the acceleration. Since when did static friction act in the direction of motion? Is it when the object is moving? And does this only apply to static friction or kinetic as well. In which types of situations is it in the opposite direction.

Homework Equations

F= mu Fn

The Attempt at a Solution

I pretty much outlined my views in the statement of the question. Please help.

 

[PhanthomJay]

Static or kinetic friction always acts opposite the direction of the relative motion or pending motion between the 2 surfaces. When a car moves forward, or when a person walks forward, it or he/she must apply a backwards force on the ground; by Newton's third law, the ground then must exert a forward force (the friction force) on the car or person. Or else neither could ever move forward.