• vivekfan
In summary, the direction of static or kinetic friction depends on the relative motion or impending motion between two surfaces. In situations where an object is moving, such as a car making a turn, static friction can act in the direction of motion to keep the object moving in a circular path. However, in most cases, friction acts in the opposite direction of motion, as seen when a person walks forward and the ground exerts a backwards force to allow for movement. The equation F= mu Fn can be used to calculate the magnitude of the friction force.
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.

F= mu Fn

## The Attempt at a Solution

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.

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!

## 1. What is static friction?

Static friction is a force that opposes the motion of an object when it is in contact with a surface and at rest. It is caused by the interaction of microscopic roughness between the two surfaces.

## 2. How is static friction different from kinetic friction?

Static friction occurs when an object is at rest, while kinetic friction occurs when an object is in motion. Additionally, the magnitude of static friction is typically greater than kinetic friction.

## 3. What factors affect the magnitude of static friction?

The magnitude of static friction is affected by the normal force between the two surfaces, the coefficient of static friction, and the roughness of the surfaces in contact.

## 4. How is the coefficient of static friction determined?

The coefficient of static friction is determined experimentally by measuring the maximum force needed to overcome static friction and cause an object to start moving, and then dividing that force by the normal force.

## 5. Can static friction be greater than kinetic friction?

Yes, static friction can be greater than kinetic friction as there is usually an extra force required to overcome the initial resistance and start an object in motion. Once the object is in motion, the force required to maintain that motion is typically less.

• Introductory Physics Homework Help
Replies
9
Views
1K
• Introductory Physics Homework Help
Replies
17
Views
794
• Introductory Physics Homework Help
Replies
61
Views
525
• Introductory Physics Homework Help
Replies
24
Views
1K
• Introductory Physics Homework Help
Replies
10
Views
2K
• Introductory Physics Homework Help
Replies
4
Views
1K
• Introductory Physics Homework Help
Replies
10
Views
2K
• Introductory Physics Homework Help
Replies
14
Views
2K
• Introductory Physics Homework Help
Replies
19
Views
1K
• Introductory Physics Homework Help
Replies
1
Views
3K