AP Physics 1: Power and Energy

Introduction to Power

• Definition: Power is the rate at which work is done or energy is transformed.
• Units: Measured in watts (W), where 1 watt equals 1 joule/second.
• Formula: Power = Change in Energy / Time (ΔE/Δt).

Scenario: Lifting a Piano

• Situation: Moving a 350 kg piano into a 4th-floor apartment using a motor.
• Energy Calculation: Gravitational potential energy given by mgh (mass × gravity × height).
  • Example: 52,000 joules of work required to lift the piano.
• Time Consideration: How power relates to the speed of doing work.
• Power Calculation:
  • Energy divided by time (300 seconds for 5 minutes) = about 175 watts.

Conversion Between Units

• Horsepower: Common non-SI unit for power from historical context (1 horsepower = 746 watts).
• Application: Converting 175 watts to about 0.23 horsepower.

Example Scenarios

Jogger on Stairs

• Details: 70 kg jogger climbs 4.5 meters in 4 seconds.
• Power Output: Approximately 788 watts.
• Energy Calculation: Power × Time = 3152 joules.

Car and Motor Power

• Example: A car with a constant 90 km/h speed and 18 horsepower output.
• Calculating Force: Power = Force × Velocity; Rearranged to find Force.
• Result: Average force exerted is approximately 537 newtons.

Classic Car Example

• Vehicle: Aston Martin DB5 from a James Bond movie.
• Specifications: Rated at 222 horsepower, acceleration from 0 to 60 mph in 8.1 seconds, mass of 1775 kg.
• Average Power: During acceleration only about 105 horsepower.

Key Takeaways

• Power Concept: How fast work is done or energy is transformed.
• Relation: From energy and time, you can calculate power and vice versa.
• Practical Understanding: Different scenarios illustrate the principles of power and energy transformations.