Kinetic Friction and Coefficients
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Questions and Answers

What is the coefficient of static friction denoted as?

  • fk
  • µk
  • µs (correct)
  • fs

    • Kinetic friction is dependent on the area of contact between surfaces.

    False

    What happens to the frictional force once the relative motion has started?

    It decreases from the static maximum value.

    The force that opposes the applied force when a body is at rest is known as __________.

    <p>static friction</p> Signup and view all the answers

    Match the types of friction with their definitions:

    <p>Static Friction = Resists the start of motion between surfaces at rest Kinetic Friction = Opposes motion between surfaces in relative motion Frictional Force = The force opposing the motion of a body in contact Coefficient of Friction = A ratio that expresses the frictional force between surfaces</p> Signup and view all the answers

    Which equation represents the law of static friction?

    <p>fs ≤ µs N</p> Signup and view all the answers

    A small applied force is always enough to move a body at rest.

    <p>False</p> Signup and view all the answers

    What is the relationship between the applied force F and the static frictional force fs when the body doesn't move?

    <p>The applied force F is equal to the static frictional force fs.</p> Signup and view all the answers

    What type of friction opposes the motion of a body that is already in motion?

    <p>Kinetic friction</p> Signup and view all the answers

    The coefficient of kinetic friction is always greater than the coefficient of static friction.

    <p>False</p> Signup and view all the answers

    What is the relationship between the applied force and the kinetic friction for a body moving at constant velocity?

    <p>F = fk</p> Signup and view all the answers

    The equation for kinetic friction is represented as fk = _____ N.

    <p>µk</p> Signup and view all the answers

    Match the type of friction with its appropriate description:

    <p>Static friction = Opposes the start of motion Kinetic friction = Opposes motion of a moving body Rolling friction = Occurs when an object rolls over a surface Fluid friction = Resists motion through a fluid</p> Signup and view all the answers

    When the external force on a body exceeds the maximum limit of static friction, what happens?

    <p>The body begins to move</p> Signup and view all the answers

    The frictional force only depends on the velocity of the bodies in contact.

    <p>False</p> Signup and view all the answers

    What is the effect on acceleration when the applied force is removed from a moving body?

    <p>The acceleration is -fk/m.</p> Signup and view all the answers

    Which coefficient is generally higher, the coefficient of static friction or the coefficient of kinetic friction?

    <p>Coefficient of static friction</p> Signup and view all the answers

    The direction of the net force acting on an object is always in the direction of its motion.

    <p>False</p> Signup and view all the answers

    What is the formula that represents the relationship between force, mass, and acceleration according to Newton's second law of motion?

    <p>F = m a</p> Signup and view all the answers

    The frictional force when a body is in motion is proportional to the coefficient of _____ and the normal force.

    <p>kinetic friction (µk)</p> Signup and view all the answers

    Match the type of friction with its description:

    <p>Static Friction = Friction when an object is at rest Kinetic Friction = Friction when an object is in motion Rolling Friction = Friction experienced by rolling objects Limiting Friction = Maximum static friction before motion begins</p> Signup and view all the answers

    When a ball is thrown upwards and reaches its maximum height, what can be said about its velocity, force, and acceleration?

    <p>Velocity is zero, force is weight, acceleration is gravitational acceleration (g)</p> Signup and view all the answers

    A stone released from an accelerated train experiences a horizontal force immediately after release.

    <p>False</p> Signup and view all the answers

    What happens to the acceleration of a body when it is momentarily at rest but experiencing net force?

    <p>The acceleration is not zero; it is determined by the net force acting on it.</p> Signup and view all the answers

    Study Notes

    Friction Fundamentals

    • Kinetic friction is nearly independent of velocity and described by the equation ( f_k = \mu_k N ), where ( \mu_k ) is the coefficient of kinetic friction.
    • The coefficient of kinetic friction, ( \mu_k ), is always less than the coefficient of static friction, ( \mu_s ).
    • When external force exceeds the maximum static friction, the object transitions to motion, subjecting it to kinetic friction.
    • If body motion is constant, the applied force equals the kinetic friction: ( F = f_k ).
    • Upon removal of the applied force, the acceleration is negative and equal to ( -f_k/m ), causing the body to eventually stop.

    Laws of Friction

    • Friction laws are empirical and approximate, not fundamental like gravitational or electromagnetic forces.
    • Frictional forces do not depend on contact area and follow these relations:
      • Static friction: ( f_s \leq f_{s , max} = \mu_s R )
      • Kinetic friction: ( f_k = \mu_k N )

    Coefficients of Friction

    • ( \mu_s ) and ( \mu_k ) are constants specific to the materials in contact.
    • Experimental findings indicate that kinetic friction coefficients ( \mu_k ) are lower than static friction coefficients ( \mu_s ).

    Key Points to Remember

    • Applied force may not always be in the direction of motion (can be opposite, normal, or at an angle).
    • A body at rest (v=0) can still experience a force (like gravity) acting on it.
    • The forces acting on a body are determined by conditions at its location, not previous motion history.
    • In the second law of motion, ( F = ma ), ( F ) represents the net external force, while ( ma ) is the result of ( F ) and should not be considered a separate force.
    • A small applied force may not be sufficient to move an object at rest due to counteracting frictional forces, leading to zero net force until the applied force overcomes static friction.

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    Description

    This quiz covers the principles of kinetic friction, including its independence from velocity and the equation governing it. Participants will explore the coefficient of kinetic friction and its application in physics. Test your understanding of these essential concepts!

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