(455) Friction [IB Physics SL/HL]

Friction Overview

• Friction is a force that opposes motion, often resulting in energy loss as heat.

  • Example: Rubbing hands together generates heat due to friction.

Forces Acting on Objects

• When a dinosaur pulls on a person:

  • Weight (W) opposes movement downward.

  • Reaction force (R), also called the normal force, acts upward.

  • Force applied by the dinosaur counters friction.

  • If applied force and friction aren't equal, acceleration occurs.

Friction Force Relationship

• Friction force (F_f) is proportional to normal force (R):F_f = μ * R

  • μ (mu) is the coefficient of friction.

Types of Friction

Static Friction

• Occurs when an object is at rest.

  • Formula: F_f < μ_s * R

• To overcome static friction, applied force must equal friction force for movement to begin.

Dynamic Friction

• Occurs when an object is moving.

  • Represented with μ_D for the coefficient of dynamic friction.

Real-World Example

• Object on a board raised at an angle (θ) until it starts sliding:

  • Weight (W) = mg (mass times gravity).

  • Normal force at angle θ = N or R.

  • Forces resolved using trigonometry:

    • Downward force: mg

    • Normal force: R = mg*cos(θ)

• To find coefficient of static friction:

  • At sliding start, static friction force (F_s) = mg*sin(θ).

  • Equation: F_s = μ_s * N

    • Set static friction force equal to gravitational force along the incline:

      • mgcos(θ) * μ_s = mgsin(θ).

  • Cancel mg:

    • Result: μ_s = sin(θ) / cos(θ) = tan(θ).

Key Takeaways

• Understand how to decompose forces into components.

• Key to solving friction problems involves identifying normal and gravitational forces.