⚙️AP Physics C: Mechanics (2025) Unit 3 – Work, Energy, and Power

Key Concepts and Definitions

• Work defined as the product of force and displacement in the direction of the force $W = \vec{F} \cdot \vec{d}$
• Energy is the capacity to do work and exists in various forms (kinetic, potential, thermal, etc.)
• Kinetic energy $KE = \frac{1}{2}mv^2$ depends on an object's mass and velocity
• Potential energy $PE = mgh$ depends on an object's mass, height, and the gravitational acceleration
• Conservative forces (gravity, springs) allow energy to be stored and recovered without loss
  • Non-conservative forces (friction, air resistance) dissipate energy as heat or other forms
• Power measures the rate at which work is done or energy is transferred $P = \frac{W}{\Delta t}$
• Joule (J) is the SI unit for work and energy, equivalent to 1 Newton-meter (N⋅m)
• Watt (W) is the SI unit for power, equivalent to 1 Joule per second (J/s)

Work and Its Mathematical Representation

• Work is a scalar quantity that quantifies the effect of a force acting over a displacement
• Positive work is done when the force and displacement are in the same direction $\theta < 90°$
• Negative work is done when the force and displacement are in opposite directions $\theta > 90°$
• No work is done when the force is perpendicular to the displacement $\theta = 90°$
• Work can be calculated using the dot product of force and displacement vectors $W = \vec{F} \cdot \vec{d} = |\vec{F}||\vec{d}|\cos\theta$
  • $\theta$ is the angle between the force and displacement vectors
• Work is a path-independent quantity; it depends only on the initial and final positions
• The work done by a variable force can be calculated using integration $W = \int_{x_1}^{x_2} F(x) dx$

Energy Types and Transformations

• Kinetic energy is the energy of motion, dependent on an object's mass and velocity $KE = \frac{1}{2}mv^2$
• Potential energy is the stored energy due to an object's position or configuration
  • Gravitational potential energy $PE_g = mgh$ depends on height in a gravitational field
  • Elastic potential energy $PE_e = \frac{1}{2}kx^2$ is stored in deformed elastic materials (springs)
• Chemical energy is stored in the bonds between atoms and molecules (fuels, batteries)
• Thermal energy is the kinetic energy of atoms and molecules due to their random motion
• Electrical energy is the potential energy of charged particles in an electric field
• Energy can be transformed from one form to another (kinetic to potential, chemical to thermal, etc.)
  • Energy transformations are governed by the conservation of energy principle

Conservation of Energy Principle

• The total energy of a closed system remains constant; energy cannot be created or destroyed
• Energy can be transferred between objects or converted from one form to another
• In the absence of non-conservative forces, the sum of kinetic and potential energy remains constant $KE_1 + PE_1 = KE_2 + PE_2$
• Work done by non-conservative forces (friction) reduces the mechanical energy of a system
  • The lost mechanical energy is converted to thermal energy or other forms
• The work-energy theorem states that the net work done on an object equals its change in kinetic energy $W_{net} = \Delta KE = \frac{1}{2}mv_f^2 - \frac{1}{2}mv_i^2$
• Conservation of energy is a fundamental principle in physics and applies to various systems (mechanical, thermodynamic, electromagnetic)

Power: Definition and Calculations

• Power is the rate at which work is done or energy is transferred $P = \frac{W}{\Delta t}$
• Instantaneous power is the limit of average power as the time interval approaches zero $P = \lim_{\Delta t \to 0} \frac{\Delta W}{\Delta t} = \frac{dW}{dt}$
• Power can also be expressed as the product of force and velocity $P = \vec{F} \cdot \vec{v}$
  • This is evident from the definitions of work and power $P = \frac{W}{\Delta t} = \frac{\vec{F} \cdot \vec{d}}{\Delta t} = \vec{F} \cdot \frac{\vec{d}}{\Delta t} = \vec{F} \cdot \vec{v}$
• The SI unit for power is the Watt (W), equivalent to 1 Joule per second (J/s)
• Other common units include horsepower (hp) and kilowatts (kW)
  • 1 hp ≈ 746 W
  • 1 kW = 1000 W

Applications in Mechanics

• Work-energy principles are used to analyze the motion of objects under the influence of forces
• The work done by a force can be used to calculate changes in an object's kinetic or potential energy
  • For example, the work done by gravity on a falling object equals its change in gravitational potential energy $W_g = -\Delta PE_g = mg(h_f - h_i)$
• Power calculations are important in designing machines and engines (cars, elevators, generators)
  • The power output of an engine determines its ability to perform work over time
• Conservation of energy is used to solve problems involving the exchange of energy between systems
  • For example, in a pendulum, energy is continuously converted between kinetic and potential forms
• Work-energy methods can simplify problem-solving by eliminating the need to analyze forces and accelerations directly

Problem-Solving Strategies

• Identify the relevant forms of energy in the system (kinetic, potential, thermal, etc.)
• Determine the initial and final states of the system, and identify any energy transformations that occur
• Apply the conservation of energy principle, equating the total energy in the initial and final states
• If non-conservative forces are present, account for the work they do and the energy they dissipate
• Use the work-energy theorem to relate changes in kinetic energy to the net work done on the system
• Break complex problems into smaller steps, and apply conservation of energy to each step separately
• Check the units of your results to ensure they are consistent with the quantities being calculated (J for energy, W for power)
• Verify that your results make sense in the context of the problem, and consider any limiting cases or special scenarios

Real-World Examples and Case Studies

• Roller coasters: Gravitational potential energy is converted to kinetic energy as the coaster descends hills, and kinetic energy is converted back to potential energy as it ascends
  • Energy is dissipated due to friction and air resistance, so the coaster never returns to its original height without additional work input
• Hydroelectric power plants: The gravitational potential energy of water is converted to kinetic energy as it flows downhill, then to electrical energy by a turbine and generator
• Automotive engines: Chemical energy stored in fuel is converted to thermal energy through combustion, then to mechanical work by the engine pistons
  • The power output of the engine determines the vehicle's acceleration and top speed
• Springs and shock absorbers: Elastic potential energy is stored in compressed or stretched springs, then released as kinetic energy when the spring returns to its equilibrium position
  • This principle is used in vehicle suspension systems and mechanical watches
• Human metabolism: Chemical energy stored in food is converted to thermal and mechanical energy by the body's cells, allowing us to maintain body temperature and perform physical work
• Solar panels: Electromagnetic energy from sunlight is converted to electrical energy by photovoltaic cells, then used to power electronic devices or stored in batteries
• Heating and cooling systems: Thermal energy is transferred from one object or substance to another, either through work done by a heat engine (air conditioner) or by direct heat transfer (furnace)