Physical Science · 4 min read
Energy, Forces, and Motion
Forms of Energy
Kinetic and Potential Energy
Kinetic energy is energy of motion, calculated as KE = ½mv². Potential energy is stored energy due to position or configuration. Gravitational potential energy equals mgh. Elastic potential energy is stored in stretched or compressed objects.
Other Energy Forms
Thermal energy relates to particle motion and temperature. Chemical energy is stored in chemical bonds. Nuclear energy is stored in atomic nuclei. Electrical energy involves moving charges. Electromagnetic energy includes light and other radiation.
Energy Transformations
Energy transforms between forms but total energy remains constant (conservation of energy). Examples include photosynthesis converting light energy to chemical energy, generators converting mechanical to electrical energy, and batteries converting chemical to electrical energy.
Thermodynamics
Laws of Thermodynamics
The zeroth law establishes thermal equilibrium: if two systems are each in equilibrium with a third, they are in equilibrium with each other.
The first law states that energy cannot be created or destroyed, only transformed. In any process, total energy remains constant. ΔU = Q – W, where U is internal energy, Q is heat added, and W is work done by the system.
The third law states that entropy approaches zero as temperature approaches absolute zero.
Heat Transfer
Conduction transfers heat through direct molecular contact. Metals conduct heat well; insulators resist conduction. Convection transfers heat through fluid movement. Warm fluid rises; cool fluid sinks. Radiation transfers heat through electromagnetic waves. No medium required; travels at light speed.
Specific heat capacity is the heat required to raise one gram of a substance by one degree Celsius. Q = mcΔT, where Q is heat, m is mass, c is specific heat, and ΔT is temperature change.
Phase Changes
Phase changes occur at constant temperature while adding or removing heat. Melting (solid to liquid) and freezing (liquid to solid) occur at the melting point. Vaporization (liquid to gas) and condensation (gas to liquid) occur at the boiling point. Sublimation is direct solid to gas transition. Deposition is direct gas to solid transition.
Heat of fusion is energy required to melt a substance. Heat of vaporization is energy required to vaporize a substance.
Forces and Motion
Newton's Laws of Motion
Types of Forces
Gravitational force attracts masses. Weight equals mg. Gravitational force decreases with distance squared.
Normal force acts perpendicular to surfaces in contact. It prevents objects from passing through surfaces.
Friction opposes relative motion between surfaces. Static friction prevents motion; kinetic friction opposes ongoing motion. Friction force equals coefficient of friction times normal force.
Tension is pulling force transmitted through strings, ropes, or cables.
Applied force is direct push or pull on an object.
Air resistance opposes motion through air, increasing with velocity.
Motion Concepts
Displacement is change in position with direction. Velocity is rate of change of displacement. Speed is magnitude of velocity. Acceleration is rate of change of velocity.
Kinematic equations describe motion with constant acceleration:
v = v₀ + at
x = x₀ + v₀t + ½at²
v² = v₀² + 2a(x – x₀)
Projectile Motion
Projectile motion combines constant horizontal velocity with vertical acceleration due to gravity. Horizontal and vertical components are independent. Trajectory is parabolic in absence of air resistance.
Circular Motion
Uniform circular motion requires centripetal acceleration directed toward center. Centripetal force equals mv²/r. Examples include satellites in orbit, cars rounding curves, and objects on rotating platforms.
Momentum and Collisions
Linear Momentum
Momentum equals mass times velocity (p = mv). Momentum is a vector quantity. Impulse equals change in momentum and equals force times time interval (J = FΔt = Δp).
Conservation of Momentum
In closed systems with no external forces, total momentum is conserved. In collisions, total momentum before equals total momentum after.
Elastic collisions conserve both momentum and kinetic energy. Inelastic collisions conserve momentum but not kinetic energy. Perfectly inelastic collisions result in objects sticking together, with maximum kinetic energy loss.
Work, Energy, and Power
Work
Work equals force times displacement times cosine of the angle between them (W = Fd cos θ). Work is done only when force causes displacement. Unit is the joule (J).
Work-Energy Theorem
Net work on an object equals change in kinetic energy. W_net = ΔKE = ½mv² – ½mv₀².
Conservation of Mechanical Energy
In absence of nonconservative forces (friction, air resistance), mechanical energy is conserved. KE + PE = constant. Energy transforms between kinetic and potential forms.
Power
Power is rate of doing work or rate of energy transfer. P = W/t = Fv. Unit is the watt (W), equal to one joule per second.
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