Titus 2/14

Types of Friction

• Definitions:

  • Static Friction: Occurs when two surfaces are not sliding past each other.

  • Kinetic Friction: Occurs when two surfaces are sliding against each other.

Static Friction

• Changes based on applied force:

  • Starts at 0 when no force is applied.

  • Increases up to a maximum value when force is applied.

• Maximum Static Friction (F_{s,max}) = μ_s * N

  • μ_s: Coefficient of static friction.

  • N: Normal force (the upward force exerted by a surface).

• The example with the brick demonstrated that static friction can adjust to counterbalance applied forces until the brick begins to slide, at which point static friction reaches its maximum value.

• Breaking Point: The threshold at which static friction can no longer hold the surfaces together, causing sliding to commence.

Normal Force (N)

• Defined as the force perpendicular to the contact surface that supports the weight of the object.

• The normal force can change based on additional vertical forces applied to the object (e.g., pushing down on an object increases its normal force).

Kinetic Friction

• Characteristics:

  • Kinetic friction has a constant magnitude and does not depend on other forces once sliding has started.

  • Its value depends on the materials in contact:

    • F_k = μ_k * N

    • μ_k: Coefficient of kinetic friction.

• Once something begins to slide, pushing with the same amount of force will not increase the friction; it remains constant based on kinetic friction coefficients.

Components of Forces

• Discusses how friction (parallel component) and normal force (perpendicular component) are part of the overall contact force directed at the object.

• The normal force and frictional force are different components of the force vector that act on the object and can change in response to other applied forces.

• A chair is an example, exerting forces upward and frictional forces opposing the applied force.

Calculating Effects of Friction

• Example Scenario: Recliner on a surface:

  • If pushing on a recliner, determine how much force is needed to overcome static friction before it slides.

  • For kinetic friction, determine the force needed to maintain constant velocity after initial movement has started.

• Requires setting up a free body diagram and listing out forces:

  • Forces acting include applied force, gravitational force, frictional forces, and normal forces.

Steps to Solve Friction Problems

1. Draw a Picture: Sketch the system.

2. Define the System: Identify the object of interest (the recliner, for instance).

3. Identify Surroundings: Include all forces acting on the object, such as applied forces and normal forces.

4. Set Up a Free Body Diagram: Keep it simple and focus on net forces.

5. Apply Momentum Principles: Use

   • Net Force = 0 for constant velocity (balances out).

   • Calculate forces in both x and y directions.

   • Remember that if the system's momentum is changing, then net forces do not equal zero.

Frictional Forces in Context

• Real-life scenarios show it is often harder to initiate motion (overcoming static friction) than to maintain motion (overcoming kinetic friction).

• The coefficients of static friction are generally higher than those of kinetic friction (μ_s > μ_k).

Conclusion on Friction Mechanics

• Coefficients of friction measure how easily materials slide against one another.

• Required calculations often revolve around finding normal forces and subsequently applying those to determine frictional forces using coefficients.

• Understanding normal force and its relationship with static and kinetic friction is crucial for solving various physics problems regarding motion.