AP Physics 1 Unit 4: Work, Energy, and Power

Introduction to Work

• Definition of Work: Work is done when a force is applied to an object, causing it to move.

  • Force must be in the direction of movement.
  • Work is only done if certain conditions are met:
    1. Force is applied: No force, no work.
    2. Motion occurs: No motion, no work.
    3. Cause: The force must cause the motion.

• Example: A man carrying a briefcase applies a vertical force but does no work if the force doesn’t cause horizontal movement.

Calculating Work

• Formula: Work = Force × Distance × cos(θ)
  • Cosine Component: Ensures force is parallel to the motion. Cosine accounts for the angle between force and motion.
  • Net Work: Sum of all forces doing work on an object.

Key Relationships in Work

• Work is directly proportional to force and distance.
• Angle Effects:
  • θ = 0: Max Work (cos(0) = 1)
  • θ = 90: Zero Work (cos(90) = 0)
  • θ between 0-90: Positive Work
  • θ between 90-180: Negative Work (commonly due to friction)

Example Calculations

Example 1: Lifting a Pumpkin

• Scenario: Lifting a pumpkin 0.80 meters.
• Calculation:
  • Work = Force × Distance.
  • Force is the weight of the pumpkin (mg).
  • Plug in values: Work = 3.2 kg × 9.8 m/s² × 0.80 m = 25 Joules.
• Units: Joules (J), equivalent to Newton-meters (N·m).

Example 2: Sliding a Drum Set

• Scenario: Push with 24 N over 1.5 meters against friction.
• Strategy: Use a Free Body Diagram to identify forces doing work (Applied Force and Friction).
• Calculation:
  • Find net force: Applied Force - Friction.
  • Friction = μ × Normal Force (μ = 0.41, Normal Force = weight = 54 N).
  • Net Work = Net Force × Distance = (24 - (0.41×54)) × 1.5 m.
  • Resulting Work ≈ 3 Joules (Net work on drum set).

Next Topics

• Upcoming: Discussion on Kinetic Energy and the Work-Energy Theorem.

These notes capture the key concepts and examples related to the physics of work, including how to calculate work and the conditions necessary for work to be done.