Overview
This lecture introduces the physics definitions of work and energy, explains how to calculate work, and outlines their relationship through the work-energy theorem.
Definitions of Work and Energy in Physics
- In physics, work occurs when a force causes displacement of an object.
- Work is calculated as the magnitude of the applied force times the displacement of the object.
- The unit of work is the joule (J), where 1 joule = 1 Newton meter (N·m).
- Work only occurs if displacement happens; pushing against an unmoving object does zero work.
Calculating Work
- Only the component of force parallel to the displacement counts toward work done.
- General formula: ( W = F d \cos \theta ), where θ is the angle between force and displacement.
- If force is parallel to displacement (θ = 0°), ( \cos \theta = 1 ), so ( W = Fd ).
- If force is perpendicular (θ = 90°), ( \cos \theta = 0 ), and no work is done.
Sign and Nature of Work
- Work is a scalar (no direction), but can be positive or negative.
- Positive work: force and displacement are in the same direction (object speeds up).
- Negative work: force opposes displacement (object slows down).
Energy and the Work-Energy Theorem
- Energy is defined as the capacity to do work.
- The work-energy theorem states: work done on an object results in a change in its kinetic energy.
- Energy can exist in many forms (kinetic, potential, thermal, chemical).
- The joule is also the unit for all forms of energy.
Key Terms & Definitions
- Work — action done when a force causes displacement; ( W = Fd ) or ( W = Fd\cos\theta )
- Joule (J) — SI unit of work and energy, equal to one Newton meter
- Energy — capacity to do work; property that can be transferred between objects or systems
- Work-Energy Theorem — states that work done on an object changes its kinetic energy
- Kinetic Energy — energy of motion
Action Items / Next Steps
- Review types of energy: kinetic, potential, thermal, chemical, etc.
- Prepare for discussion of more energy forms in upcoming lectures.