Overview
This lecture covers the fundamental concepts of work, energy, and power in physics, including formulas, calculations, and typical exam questions.
Work
- Work is the product of force and displacement in the direction of the force.
- It is a scalar quantity measured in Joules (J).
- Formula: ( W = F \times D ) where ( F ) is force (Newtons) and ( D ) is displacement (meters).
- Work is positive when force and displacement are in the same direction, negative when in opposite directions, and zero if they are perpendicular.
- When force is at an angle, only the horizontal component (( F \cos\theta )) does work.
Work Done by Gases
- For gas in a cylinder, ( W = P \Delta V ), where ( P ) is pressure and ( \Delta V ) is the change in volume.
Types of Energy
- Energy is the ability to do work; measured in Joules.
- Main forms: kinetic, gravitational potential, elastic (strain), chemical, electrical, thermal, radiation, sound, and nuclear energy.
Conservation of Energy
- Energy cannot be created or destroyed, only transferred between forms.
Work-Energy Principle
- Work done is equal to energy transferred.
- Kinetic Energy: ( E_K = \frac{1}{2} m v^2 )
- Gravitational Potential Energy: ( E_P = m g \Delta h )
Power
- Power is the rate of work done or energy transferred per unit time.
- Formula: ( P = \frac{W}{t} = \frac{E}{t} )
- If object moves with velocity ( V ): ( P = F \times V )
- Units: Watts (W)
Efficiency
- Efficiency = ( \frac{\text{useful output}}{\text{total input}} )
- Can be calculated using energy or power, expressed as a ratio or percentage.
Example Problems
- Calculating work done given force and distance.
- Calculating power required for lifting/transporting objects.
- Determining efficiency of systems (e.g., hydroelectric, bow and arrow).
- Using conservation of energy to compute speeds or energy changes.
Key Terms & Definitions
- Work (W) — Product of force and displacement in the same direction.
- Energy — Capacity to do work; scalar with unit Joule (J).
- Power (P) — Work done or energy transferred per unit time; unit is Watt (W).
- Efficiency — Ratio of useful output to total input, often given in percent.
- Kinetic Energy (E_K) — Energy due to motion: (\frac{1}{2} m v^2).
- Potential Energy (E_P) — Energy due to position: ( m g \Delta h ).
- Joule (J) — SI unit of work or energy.
Action Items / Next Steps
- Review and solve similar exam-style questions in your textbook.
- Memorize key formulas for work, energy, and power.
- Practice calculating efficiency and interpreting results in context.