Friction

Friction

Page 1

  • Friction: A force that resists the relative motion of solid surfaces, fluid layers, and material elements sliding against each other.

Page 2

  • Do Now Activities:

    • Scenario: My three brothers and I are fighting over the last box of cereal, initially at rest.

    • Calculations Required:

      • Fnet: Net Force

      • Acceleration (a): Change in Velocity per unit time

      • Velocity (v) after 5 seconds

      • Displacement (Dx) after 5 seconds

    • Given Data:

      • Mass = 20 kg

      • F1 = 100 N, F2 = 55 N, F3 = 30 N, F4 = 85 N

Page 3

  • Concepts of Friction:

    • Static Scenario: An object remains at rest despite the application of force, indicating the force is insufficient to overcome friction.

    • Moving Scenario: An object begins to move when the applied force is large enough to overcome friction.

Page 4

  • Frictional Force:

    • A force opposing the motion of an object due to contact between surfaces.

    • Coefficients of Friction: A numerical representation of surface roughness/smoothness.

    • Normal Force (FN): The upward force exerted by a surface against an object resting on it.

Page 5

  • Coefficient of Friction (µ):

    • Independent of surface area; determined by the textures of the contacting surfaces.

Page 6

  • Characteristics of the Coefficient of Friction (µ):

    • Unitless Number: Ranges from 0 (perfectly frictionless) to 1 (very high friction).

Page 7

  • Types of Friction:

    • Static Friction: Friction present before motion starts.

    • Kinetic Friction: Friction that occurs during the sliding of two surfaces against each other.

Page 8

  • Comparative Frictional Forces:

    • Static friction typically > kinetic friction (µStatic (S) > µKinetic (K)).

    • It is generally more difficult to start moving an object than to keep it in motion.

Page 9

  • Force of Friction:

    • Formula: Ff = µFN

      • Where:

        • Ff = Force of Friction

        • µ = Coefficient of Friction

        • FN = Normal Force

Page 10

  • Net Force Calculation:

    • Formula: Fnet = Fpull/push – Ff

    • To Determine Net Force: Subtract the frictional force from the applied force.

Page 11

  • Static and Kinetic Friction Characteristics:

    • For Static Friction: Ff < µsFN; Ff = Fpull

    • For Kinetic Friction (Constant Velocity): Ff = µKFN; Ff = Fpull

    • For Kinetic Friction (Accelerating): Ff = µKFN; Ff < Fpull

Page 12

  • Free Body Diagrams with Friction:

    • Diagram includes:

      • Upward Force (FN)

      • Applied Force (Fpull)

      • Frictional Force (Ff)

      • Gravitational Force (Fg)

Page 13

  • Example 1:

    • Given: Fpull = 30 N

    • Object mass = 100 kg

    • Questions:

      • a.) Calculate weight of the object

      • b.) Determine the force of friction

      • c.) Find the coefficient of kinetic friction

      • d.) Identify the combination of surfaces via reference table.

Page 14

  • Example 2:

    • Given: Fpull = 40 N, Mass = 8 kg, Acceleration = 4 m/s²

    • Questions:

      • a.) Calculate Fnet

      • b.) Determine the force of friction

      • c.) Find the coefficient of friction.

Page 15

  • Example 3:

    • Given: 5 kg box, 27 N force to the East, 17 N friction to the West

    • Question: What is the magnitude of the acceleration of the box?

Page 16

  • Example 4:

    • Given: 8.0 N wooden block moving at constant velocity

    • Question: What is the magnitude of the kinetic friction between the block and the floor?

Page 17

  • Example 5:

    • Given: 4.0 kg object accelerating at 10 m/s² on rough surface

    • Question: What is the magnitude of the frictional force acting on the object?

Page 18

  • Example 6:

    • Given: 300 kg crate, 380 N forward push, 350 N pull

    • Question: What is the crate’s coefficient of kinetic friction on the floor (moving at constant speed)?