2.7 Kinetic and Static Friction

• Kinetic friction is the force that opposes the motion of an object sliding across a surface
• Kinetic friction acts in the same direction as motion

  False
• In which direction does kinetic friction act?

  Opposite to motion
• Static friction acts to prevent an object from starting to move.


  True
• What is the typical relationship between the coefficients of static and kinetic friction?

  $\mu_{s} > \mu_{k}$
• The value of $\mu_{k}$ is usually less than the coefficient of static friction, $\mu_{s}$ because it requires less force to maintain motion
• Match the aspect of kinetic friction with its description:

  Definition ↔️ Opposes the motion of sliding objects

  Formula ↔️ $f_{k} =$$\mu_{k} N$

  Direction ↔️ Opposite to the direction of motion

  Coefficient ↔️ $\mu_{k}$
• Static friction is the force that opposes the motion of an object when it is at rest
• Static friction prevents an object from starting to move


  True
• The coefficient of static friction, $\mu_{s}$, is usually greater than the coefficient of kinetic friction, $\mu_{k}$, because it requires more force to overcome inertia
• Match the aspect of static friction with its description:

  Definition ↔️ Opposes motion of stationary objects

  Formula ↔️ $f_{s} =$$\mu_{s} N$

  Coefficient ↔️ $\mu_{s}$

  Direction ↔️ Prevents motion
• The coefficient of static friction is typically less than the coefficient of kinetic friction.

  False
• Order the following steps in solving a problem involving kinetic friction:

  1️⃣ Define the problem and identify known variables

  2️⃣ Draw a free-body diagram

  3️⃣ Apply Newton's second law

  4️⃣ Calculate the normal force

  5️⃣ Use the formula $f_{k} =$$\mu_{k} N$ to find the friction force
• Static friction opposes the motion of stationary objects, whereas kinetic friction opposes the motion of sliding objects.
• What is the definition of kinetic friction?

  Force opposing sliding motion
• The coefficient of kinetic friction is usually less than the coefficient of static friction.

  True
• Match the type of friction with its definition:

  Kinetic Friction ↔️ Opposes motion of sliding objects

  Static Friction ↔️ Opposes motion of stationary objects
• What is the definition of static friction?

  Force preventing initial motion
• What does the coefficient of friction depend on?

  Material properties and surface conditions
• Smoother surfaces have higher coefficients of friction

  False
• How does the typical value of static friction compare to kinetic friction?

  Greater than μ_k
• Match the type of friction with its formula:

  Static Friction ↔️ $f_{s} ≤ μ_{s} N$

  Kinetic Friction ↔️ $f_{k} =$$μ_{k} N$
• More energy is required to overcome the initial "grip" in static friction than in kinetic friction


  True
• What is the coefficient of friction denoted as?

  μ
• A higher coefficient of friction means a higher friction force


  True
• The typical value of kinetic friction is greater than static friction

  False
• What is kinetic friction described as?

  Force opposing sliding motion
• The normal force is a factor in the formula for kinetic friction

  True
• The value of $\mu_{k}$ is usually less than the coefficient of static friction, $\mu_{s}$ because less energy is required to maintain motion
• What is the primary difference between static and kinetic friction in terms of definition?

  Stationary vs. sliding objects
• What is static friction defined as?

  Force opposing stationary objects
• Static friction acts to prevent an object from starting to move


  True
• Match the type of friction with its definition:

  Static Friction ↔️ Opposes motion of stationary objects

  Kinetic Friction ↔️ Opposes motion of sliding objects
• Friction opposes the motion of sliding
• Arrange the following aspects to compare static and kinetic friction:

  1️⃣ Definition

  2️⃣ Formula

  3️⃣ Coefficient

  4️⃣ Direction
• Match the definitions with the type of friction:

  Opposes the motion of sliding objects ↔️ Kinetic friction

  Opposes an object from starting to move ↔️ Static friction
• A higher normal force results in greater friction.
• The typical value of the coefficient of static friction is greater than that of kinetic friction.
• What type of surfaces have lower coefficients of friction?

  Smoother surfaces
• The friction force is directly proportional to the coefficient of friction


  True