2.7 Kinetic and Static Friction

• What is the formula for the magnitude of static friction?

  $F_{s} ≤ μ_{s} N$
• Kinetic friction is the force that resists the motion of an object sliding
• What is the formula for the magnitude of kinetic friction?

  F_k = μ_k N</latex>
• Steps to calculate friction in a problem:

  1️⃣ Identify the type of friction (static or kinetic)

  2️⃣ Determine the normal force acting on the object

  3️⃣ Find the appropriate friction coefficient (μ_s or μ_k)

  4️⃣ Apply the relevant friction formula
• What is the formula for the magnitude of kinetic friction?

  F_k = μ_k N</latex>
• What happens to the magnitude of kinetic friction as long as the object is moving?

  It remains constant
• Under what condition does static friction act on an object?

  When the object is at rest
• What is the maximum static friction required to start moving a 5 kg block on a wooden floor with μ_s = 0.4?

  19.6 N
• What is the effect of increasing the normal force on static friction?

  Increases static friction
• What is the primary role of static friction?

  Prevents motion
• Static friction opposes the applied force to keep an object stationary


  True
• Static friction is the force that prevents an object from starting to move
• For a 5 kg box with μ_s = 0.4, the maximum static friction is 19.6 N
• The formula for kinetic friction is F_k = μ_k N
• Match the friction type with its properties:

  Static Friction ↔️ Object at rest

  Kinetic Friction ↔️ Object in motion
• In summary, what does static friction oppose?

  Initial applied force
• What happens to static friction if the normal force increases?

  It increases
• The kinetic friction coefficient represents the relative roughness of two surfaces in contact
• Why is the static friction coefficient typically greater than the kinetic friction coefficient?

  Harder to initiate motion
• What is the example value of the static friction coefficient for wood on wood?

  0.4
• To keep the same 10 kg box moving with μ_k = 0.3, a force of 29.4 N is required.
• Static friction varies with the applied force until it reaches its maximum value.


  True
• The static friction coefficient represents the roughness between surfaces.
• For a 5 kg box on a wooden floor with μ_s = 0.4, the maximum static friction is 19.6 N.
• What happens when the applied force exceeds the maximum static friction?

  Object starts to move
• Static friction can exceed the maximum static friction when the applied force is large enough.

  False
• Kinetic friction has a constant magnitude once the object is in motion, regardless of the applied force.


  True
• The formula for kinetic friction is F_k = μ_k N
• Under what condition does static friction act?

  Object is at rest
• What is the maximum static friction for a 5 kg block on a wooden floor with μ_s = 0.4?

  19.6 N
• A higher static friction coefficient indicates rougher surfaces.
• What two factors affect the magnitude of kinetic friction?

  μ_k and N
• What does the static friction coefficient represent?

  Ratio of F_s to N
• What is the kinetic friction force acting on the same box once it is sliding with μ_k = 0.3?

  29.4 N
• Static friction opposes the applied force, keeping the object stationary until the applied force exceeds the maximum static friction.


  True
• What is the maximum static friction for a 5 kg box on a wooden floor with μ_s = 0.4?

  19.6 N
• Kinetic friction depends on the applied force and increases with it.

  False
• To start moving a 5 kg block on a wooden floor with μ_s = 0.4, you need to overcome a maximum static friction of 19.6 N
• If a 5 kg box slides across a wooden floor with μ_k = 0.3, the kinetic friction is 14.7 N
• Kinetic friction remains constant as long as the object is in motion.


  True