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Static and Kinetic Friction: Essential Forces in Physics
Dive into the world of static and kinetic friction. Understand how these forces shape our physical world, from everyday activities to complex engineering. Master key concepts and problem-solving techniques.

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Now Playing:Friction static and kinetic– Example 0
Intros
  1. Introduction to friction
    1. What causes friction?
    2. What is kinetic friction?
    3. What is static friction?
Examples
  1. Understanding kinetic vs. static friction

    Joe wants to pull his 136 kg fridge away from the wall to clean behind it. When he pulls on the fridge with a horizontal force of 155 N, it doesn't move.

    i. What is the force of friction that acts on the fridge? What type of friction is this?

    ii. If the coefficient of friction between the fridge and floor is 0.200, what is the maximum amount of static friction that can act on the fridge?

    iii. Joe gets Samantha to pull with an additional 155 N and the fridge begins to accelerate at 0.400 m/s2m/s^{2}. What is the coefficient of friction between the fridge and the ground? What type of friction is this?
    Friction: Static and kinetic
    Jump to:NotesConceptExampleFAQsPrerequisites
    Notes
    In this lesson, we will learn:
    • Meaning of static and kinetic friction
    • When to apply static and kinetic friction to physics problems
    • Solving friction problems

    Notes:

    • Friction is the force that resists the motion of an object sliding (or trying to slide) over a surface.
    • Kinetic friction is the friction that acts when an object slides across a surface.
      • It is experimentally observed that kinetic friction is proportional to the force of the object pressing into the surface it slides across. Kinetic friction also depends on how well the two surfaces in contact grip each other.
        • This is expressed with the equation F⃗k=μkF⃗N\vec{F}_{k} = \mu_{k}\vec{F}_{N}. We can see in this equation that if the normal force acting on the object increases (due to the object pushing harder on the surface it slides on), friction force also increases proportionally.
        • μk\mu_{k} is the coefficient of kinetic friction, which tells us about the relationship between normal force and kinetic friction for two surfaces sliding past each other.
      • Coefficients of friction are dimensionless: they do not have a unit.
      • The larger the coefficient of kinetic friction, the stronger the friction between the two surfaces. A typical range for coefficient of friction values is between 0 and 1.
        • A "frictionless" surface has a coefficient of friction of 0, but in real life all surfaces have some amount of friction. Teflon, the coating of non-stick pans and one of the lowest-friction materials known, has a coefficient of friction of ~0.04.
        • Coefficients of friction greater than 1 are possible. They indicate the force of friction is greater than the normal force acting on the object. For example, some rubbers used for high-friction purposes (like rain boot soles) can have a coefficient of friction of greater than 1 with the ground.
    • Static friction is the friction that acts when a force is applied to an object, but the applied force isn't large enough to overcome the friction between the object and surface it is on.
      • Like kinetic friction, the maximum static friction is proportional to the normal force acting on the object. However, static friction only acts on an object in response to an applied force, and balances the applied force so the object does not move.
      • If applied force on an object exceeds the maximum static friction, it begins to move. Once the object is moving, the friction acting on it is kinetic friction.
      • For an object in contact with a surface, the coefficient of static friction (μs\mu_{s}) is in almost all cases larger than the coefficient of kinetic friction. As a result, the force required to get an object moving is somewhat larger than the force required to keep it in motion.
    Kinetic friction

    F⃗k=μkF⃗N\vec{F}_{k} = \mu_{k} \vec{F}_{N}

    F⃗k\vec{F}_{k}: Kinetic force of friction, in newtons (N)

    μk:\mu_{k}: Coefficient of kinetic friction

    F⃗N:\vec{F}_{N}: Normal force, in newtons (N)


    Static friction

    F⃗s≤μsF⃗N\vec{F}_{s} \leq \mu_{s}\vec{F}_{N}

    F⃗s(max)=μsF⃗N\vec{F}_{s (max)} = \mu_{s}\vec{F}_{N}

    F⃗s:\vec{F}_{s}: Static force of friction, in newtons (N)

    μs:\mu_{s}: Coefficient of static friction

    F⃗s(max):\vec{F}_{s (max)}: Maximum static force of friction, in newtons (N)

    Concept

    Introduction

    Friction is a fundamental force in physics that resists the relative motion of objects in contact. This article begins with an introductory video explaining the basics of friction, setting the stage for a deeper exploration. Understanding friction is crucial in physics, as it plays a vital role in everyday life and various scientific applications. There are two main types of friction: static friction and kinetic friction. Static friction prevents objects at rest from moving, while kinetic friction acts on objects already in motion. These concepts are essential for comprehending how objects interact in the physical world. From the tires gripping the road to the wear and tear on machinery, friction influences countless aspects of our lives. By grasping the principles of static and kinetic friction, students and enthusiasts can better analyze and predict the behavior of objects in various scenarios, making this knowledge invaluable in fields ranging from engineering to sports science.

    Example

    Question: A crate is pushed across the floor. The coefficient of friction between the crate and the floor is 0.240. If 167 N of applied force is required to keep the crate moving forwards at a constant velocity, find the mass of the crate.

    Step 1: Understand the Problem

    In this problem, we are given a crate being pushed across a floor with a coefficient of friction of 0.240. The applied force required to keep the crate moving at a constant velocity is 167 N. Our goal is to find the mass of the crate. Since the crate is moving at a constant velocity, we know that the net force acting on it is zero, meaning the applied force is balanced by the frictional force.

    Step 2: Draw a Free Body Diagram

    Start by drawing a free body diagram of the crate. This diagram should include all the forces acting on the crate:

    • An applied force of 167 N to the right.
    • A frictional force acting to the left, opposing the motion.
    • The gravitational force acting downwards, which is the weight of the crate.
    • The normal force acting upwards, which is the reaction force from the floor.

    Step 3: Identify the Type of Friction

    Since the crate is moving at a constant velocity, the type of friction involved is kinetic friction. The kinetic friction force (Fk) can be calculated using the formula:

    Fk = μk * N

    where μk is the coefficient of kinetic friction (0.240) and N is the normal force.

    Step 4: Apply Newton's Second Law

    According to Newton's second law, the net force acting on the crate is zero because it is moving at a constant velocity. This means the applied force is equal to the frictional force:

    Fapplied = Fk

    Given that Fapplied is 167 N, we can set up the equation:

    167 N = μk * N

    Step 5: Calculate the Normal Force

    Rearrange the equation to solve for the normal force (N):

    N = Fapplied / μk

    Substitute the given values:

    N = 167 N / 0.240

    Calculate the normal force.

    Step 6: Relate Normal Force to Weight

    The normal force (N) is equal to the weight of the crate (W), which is the mass (m) of the crate times the acceleration due to gravity (g):

    N = m * g

    We know that g is approximately 9.81 m/s². So, we can set up the equation:

    m * 9.81 m/s² = N

    Step 7: Solve for the Mass

    Rearrange the equation to solve for the mass (m):

    m = N / 9.81 m/s²

    Substitute the value of the normal force calculated in Step 5 into this equation to find the mass of the crate.

    Conclusion

    By following these steps, you can determine the mass of the crate based on the given coefficient of friction and the applied force required to keep it moving at a constant velocity. This method involves understanding the forces at play, applying Newton's second law, and using the relationship between normal force and weight to find the mass.

    FAQs

    1. What is the difference between static and kinetic friction?

      Static friction prevents objects at rest from starting to move, while kinetic friction opposes the motion of objects already moving. Static friction is typically greater than kinetic friction, which is why it's often harder to start moving an object than to keep it in motion.

    2. How is the coefficient of friction calculated?

      The coefficient of friction (μ) is calculated by dividing the friction force (F) by the normal force (N): μ = F / N. For static friction, we use μs, and for kinetic friction, we use μk. These coefficients are typically determined experimentally and are specific to the materials in contact.

    3. Can friction ever be beneficial?

      Yes, friction is often beneficial. It allows us to walk without slipping, enables vehicles to accelerate and brake, and is essential for writing with pens and pencils. In many mechanical systems, a certain amount of friction is necessary for proper functioning.

    4. How do lubricants reduce friction?

      Lubricants reduce friction by creating a thin film between moving surfaces, minimizing direct contact. This film allows the surfaces to slide past each other more easily, reducing wear and heat generation. Common lubricants include oils, greases, and synthetic materials designed for specific applications.

    5. What role does friction play in sports?

      Friction plays a crucial role in many sports. In activities like basketball or soccer, high friction between shoes and the playing surface provides traction for quick movements and changes in direction. Conversely, sports like skiing or ice skating rely on low friction for smooth gliding. Equipment design often focuses on optimizing friction levels for peak performance in each sport.

    Prerequisites

    Understanding the concept of friction, particularly static and kinetic friction, is crucial in physics and everyday life. While there are no specific prerequisite topics listed for this subject, it's important to recognize that a solid foundation in basic physics principles can greatly enhance your comprehension of friction. Concepts such as force, motion, and Newton's laws of motion are inherently related to the study of friction and can provide valuable context.

    Friction is a force that resists the relative motion of objects in contact. To fully grasp the nuances of static and kinetic friction, students should be comfortable with the idea of forces acting on objects. This understanding helps in visualizing how friction operates in various scenarios, from a book resting on a table to a car accelerating on a road.

    The distinction between static and kinetic friction is a key aspect of this topic. Static friction comes into play when objects are at rest relative to each other, while kinetic friction occurs when objects are in motion. This differentiation is essential for solving problems involving friction and predicting object behavior in different situations.

    Although not explicitly listed as prerequisites, familiarity with concepts like normal force and the coefficient of friction can significantly aid in understanding friction. The normal force, which is perpendicular to the surface of contact, plays a crucial role in determining the magnitude of friction. Meanwhile, the coefficient of friction, a property of the materials in contact, helps quantify the strength of frictional forces.

    Vector analysis and basic trigonometry can also be beneficial when studying friction, especially when dealing with inclined planes or complex force diagrams. These mathematical tools allow for the accurate representation and calculation of frictional forces in various orientations.

    Moreover, an understanding of energy and work can provide a broader perspective on friction's role in mechanical systems. Friction often leads to energy dissipation, which is a critical consideration in many practical applications, from automotive design to industrial machinery.

    While these topics are not listed as formal prerequisites, they form a valuable knowledge base that can enhance your understanding of static and kinetic friction. By revisiting or strengthening your grasp on these fundamental concepts, you'll be better equipped to tackle the intricacies of friction in both theoretical and practical contexts.

    Remember, the study of friction is not isolated but interconnected with various physics principles. As you delve into static and kinetic friction, you'll likely find yourself drawing upon knowledge from multiple areas of physics, reinforcing the importance of a well-rounded understanding of basic physical concepts.