Notes on Work and Energy in Physics

Introduction to Work

• Common perceptions of work: cubicles, briefcases, exams.
• Physicists define work specifically: applying a force over a distance.
• Link to conservation of energy.

Defining Work

• Work occurs when a force is applied to a system over a distance.
• Example: Dragging a box with a rope.
  • Work = Force (N) x Distance (m).
  • Units of work: Joules (J).

Work Calculation with Angled Forces

• If the force is at an angle, separate it into components:
  • Horizontal component: F * cos(θ).
• Work equation: Work = Force x Distance x cos(θ).

Non-Constant Forces

• If force varies, calculate work using integration.
• Work is a measure of energy change.

Energy

• Work and energy share the same units (Joules).
• Energy is defined as the ability to do work.

Types of Energy

• Kinetic Energy (KE): Energy of motion.
  • KE = 1/2 * mass (kg) * velocity^2 (m/s^2).
  • Example: Box with mass 20 kg, velocity 4 m/s → KE = 160 J.
• Potential Energy (PE): Energy that could be used for work.
  • Gravitational Potential Energy: PE = mass (kg) * g (9.8 m/s²) * height (m).
  • Example: Book held 1 meter → PE = 9.8 J.
  • Spring Potential Energy: PE = 1/2 * k (N/m) * distance² (m).

Energy Changes in Systems

• Non-Conservative Systems: Lose energy (e.g., through friction).
• Conservative Systems: Energy is conserved (e.g., pendulum).
  • Potential and kinetic energy exchange without loss.

Power in Physics

• Average Power: Work done over time.
  • Measured in watts (W), where 1 W = 1 J/s.
• Power equations:
  • Power = Work / Time.
  • Power = Force x Average Velocity.
• Example: Moving the box with 250 J of work over 2 seconds → Average Power = 125 W.

Conclusion

• Key concepts covered:
  • Definitions and calculations of work.
  • Energy types: kinetic and potential.
  • Non-conservative vs. conservative systems.
  • Average power and its equations.

Production Credits

• Produced by Crash Course in association with PBS Digital Studios.