Overview
Summary notes on fundamental physics concepts: forces and motion, gravity, energy and thermodynamics, electricity and magnetism, atomic and nuclear physics, relativity, and quantum mechanics.
Newtonian Mechanics
- Force is a push or pull in a direction; defined by F = m·a.
- Mass measures amount of matter and inertia; fixed mass yields predictable acceleration for a given force.
- Acceleration is how fast velocity changes over time.
- With all forces known, motion is predictable (e.g., basketball trajectory).
- Newton’s First Law: objects in motion stay in motion unless acted on by a force.
Gravity and Orbits
- Law of Universal Gravitation: gravitational pull depends on masses and distance, scaled by a constant.
- Inverse-square law: force decreases with square of distance increase.
- Planets orbit the sun due to gravity providing centripetal force and initial velocity.
- Orbits are generally elliptical; Pluto’s orbit is notably eccentric.
- Mass vs weight: mass is amount of matter; weight is gravitational force on that mass.
- Same mass on Earth and Moon; weight differs due to different gravitational pulls.
Energy, Work, and Conservation
- Energy is a scalar quantity measured in Joules; no direction component.
- Two main forms: kinetic (motion) and potential (stored due to configuration).
- Gravitational potential converts to kinetic during a fall; impact energy damages objects.
- Work is force applied over a distance; measured in Joules.
| Quantity | Meaning | Unit | Key Relation/Note |
|---|
| Force (F) | Push/pull in a direction | Newton (N) | F = m·a |
| Mass (m) | Amount of matter; inertia | kg | Invariant across location |
| Acceleration (a) | Rate of change of velocity | m/s² | From net force |
| Energy (E) | Capacity to do work | Joule (J) | Conserved; converts forms |
| Kinetic Energy | Energy of motion | J | Increases with speed |
| Potential Energy | Stored energy (e.g., gravitational) | J | Converts when configuration changes |
| Work (W) | Force over distance | J | W = F·d; zero if no displacement |
| Weight | Gravitational force on mass | N | Depends on local gravity |
- Energy vs work: energy is potential to do work; work is energy transfer via force over distance.
- No displacement means no work done, despite perceived effort.
- Conservation of energy: energy cannot be created or destroyed, only transformed.
Heat, Temperature, and Thermodynamics
- Braking converts car’s kinetic energy to heat via friction; air molecules move faster.
- Temperature is average kinetic energy of atoms in a system.
- Atoms constantly vibrate; faster motion means higher temperature.
- Entropy measures disorder and number of microstates; tends to increase.
- Ice (ordered) melting to water (disordered) increases entropy.
- Time’s arrow linked to increasing entropy; higher-entropy energy is less useful for work.
- Refrigeration lowers water’s entropy but increases room’s entropy more; total entropy rises.
Electricity and Magnetism
- Charge can be positive, negative, or neutral; electrons carry negative charge.
- Electric current is electron flow; described by current, voltage, and resistance.
- Current: charge flow rate; Voltage: electric potential difference; Resistance: opposition to flow.
- Coulomb’s Law parallels gravity: like charges repel, opposites attract.
- Electric charges produce electric fields; field lines show force direction on a test charge.
- Magnetic monopoles do not exist; poles come in north–south pairs.
- Changing magnetic fields create electric fields; moving charges/fields create magnetic fields.
- Induction: moving a magnet near a conductor generates current; enables wireless charging.
- Accelerating charges emit electromagnetic waves; frequency determines spectrum (light, Bluetooth, etc.).
- Light is the visible portion of the electromagnetic spectrum to human eyes.
Atomic and Nuclear Structure
- Molecules are made of atoms; atoms have a nucleus (protons, neutrons) and electrons.
- Protons and neutrons are made of quarks; electrons have heavier cousins.
- The Standard Model lists fundamental particles currently known.
- Element identity set by proton count; isotopes vary by neutron count.
| Component | Constituents | Notes |
|---|
| Atom | Nucleus + electrons | Basis of elements |
| Nucleus | Protons + neutrons | Defines element (protons) and isotope (neutrons) |
| Proton/Neutron | Quarks | Composite particles |
| Electron | Fundamental lepton | Has heavier relatives |
- Unstable isotopes undergo decay, emitting ionizing radiation; hazardous to health.
- Half-life: time for half of a sample of atoms to decay; varies by isotope.
Light and Relativity
- Speed of light in vacuum: 299,792,458 m/s; fastest known speed.
- Double-slit interference shows light’s wave behavior; waves add or cancel.
- Photoelectric effect: light also behaves as particles (photons).
- Special relativity assumptions: speed of light constant; laws of physics same for all observers.
- Time dilation: to keep light speed constant, moving clocks run slower relative to stationary observers.
- General relativity: gravity arises from masses curving spacetime; objects follow geodesics.
- Curved surfaces make straight paths converge (analogy: walking north from opposite coasts).
- Mass–energy equivalence explains large energy from small mass changes in nuclear processes.
Nuclear Energy: Fission and Fusion
- Fission splits heavy nuclei (often via neutron bombardment), releasing energy.
- Fusion combines light nuclei into heavier ones, releasing energy.
- Mass defect: products have less mass than reactants; missing mass converts to energy.
| Process | What Happens | Energy Source | Notes |
|---|
| Fission | Heavy nucleus splits | Mass defect (E=mc²) | Used in bombs/reactors; chain reactions |
| Fusion | Light nuclei merge | Mass defect (E=mc²) | Powers stars; challenging to control |
Quantum Mechanics
- Energy is quantized; comes in discrete packets (quanta).
- Electrons exist in superposition: multiple states until measured.
- Schrödinger equation gives probabilities; electron location is a probability cloud.
- Measurement outcomes are inherently probabilistic; exact result is random.
- Heisenberg uncertainty: cannot know exact position and exact speed simultaneously.
- Single-photon double-slit still yields interference over time; each photon interferes with itself.
- Measuring which slit destroys interference; observation collapses superposition.
Key Terms & Definitions
- Force: interaction causing acceleration; vector quantity.
- Mass: amount of matter; inertia measure.
- Acceleration: change rate of velocity over time.
- Weight: gravitational force on a mass.
- Energy: capacity to do work; conserved scalar.
- Work: force applied over distance; energy transfer.
- Temperature: average kinetic energy of particles.
- Entropy: measure of disorder and microstates; tends to increase.
- Charge: property causing electric interaction; positive/negative.
- Current: rate of charge flow.
- Voltage: electric potential difference driving current.
- Resistance: opposition to current.
- Electromagnetic wave: propagating electric and magnetic fields; includes light.
- Isotope: atoms of same element with different neutron numbers.
- Half-life: time for half a radioactive sample to decay.
- Photon: quantum of light.
- Superposition: coexistence of multiple states until measurement.
- Uncertainty principle: limits simultaneous knowledge of position and momentum.
Action Items / Next Steps
- Practice applying F = m·a and work-energy concepts to sample problems.
- Compare mass and weight in scenarios on Earth vs Moon.
- Sketch field lines for simple charge and magnet configurations.
- Analyze double-slit outcomes with and without measurement for wave–particle duality understanding.