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.

 

[nlightner]

Hello,

Thank you for your question. Your teacher is correct, static friction can act in the direction of motion in certain situations. It is important to understand that static friction is a type of force that opposes the relative motion between two surfaces in contact. This means that it can act in different directions depending on the specific circumstances.

In the case of the ball in your teacher's hand, the static friction was acting in the direction of motion because the hand was continuously applying a force to keep the ball stationary. In this situation, the static friction force is necessary to counteract the force of the hand and maintain the ball's position.

In the case of centripetal acceleration, the force of static friction is necessary to keep an object moving in a circular path. This is because the object is constantly changing direction, and the static friction force is needed to keep it from sliding off the circular path.

However, in other situations where an object is moving in a straight line, the static friction force will act in the opposite direction of the motion, as you mentioned. This is because the force of static friction is still trying to prevent relative motion between the two surfaces.

Kinetic friction, on the other hand, always acts in the direction opposite of the motion, regardless of the situation. This is because kinetic friction is the force that opposes the actual motion between two surfaces.

I hope this helps clarify your understanding of static friction. If you have any further questions, please don't hesitate to ask. Keep up the curiosity and keep learning!