Physics 71_1

• From Stanford Encyclopedia of Philosophy:

  Physics, and natural science in general, is a reasonable enterprise based on valid experimental evidence, criticism, and rational discussion. It provides us with knowledge of the physical world, and it is an experiment that provides the evidence that grounds this knowledge.
• Experiment plays many roles in science:

  - Primarily, experiments are designed to test theories and to provide the basis for scientific knowledge.
• Experiment plays many roles in science:

  • It can also call for a new theory, either by showing that an accepted theory is incorrect, or by exhibiting a new phenomenon that is in need of explanation.
• Experiment plays many roles in science:

  • Experiments can provide hints toward the structure or mathematical form of a theory and it can provide evidence for the existence of the entities involved in our theories.
• Experiment plays many roles in science:

  • May investigate a phenomenon just because it looks interesting. Such experiments may provide evidence for a future theory to explain.
• 1687: Publication of Newtons Principia
• 1820: Ørsteds discovery of the magnetic properties of electric current
• 1864: Maxwells electrodynamics
• 1870: Development of statistical mechanics
• 1925: Birth of quantum mechanics
• Classical mechanics works well for objects moving at speeds much slower than the speed of light and sizes much larger than atomic and subatomic scales.
• Relativistic Mechanics extends classical mechanics to describe the motion of objects at speeds comparable to the speed of light.
• Quantum mechanics is a fundamental theory to describe the behavior of matter and energy at the atomic and subatomic scales.
• Quantum field theory is used to describe the behavior of elementary particles and their interactions.
• physical quantity

  • is a physical property of a material or system that can be quantified by measurement
  • possesses at least two characteristics in common: numerical magnitude and units.
• The SI defines seven fundamental units, defining the others using

  these definitions. These seven fundamental units are defined from natural constants experimentally measured.
• Symbol-Name-Quantity

  s-second-time
• Symbol-Name-Quantity

  m-metre-length
• Symbol-Name-Quantity

  kg-kilogram-mass
• Symbol-Name-Quantity

  A-ampere-electric current
• Symbol-Name-Quantity

  K-kelvin-thermodynamic temperature
• Symbol-Name-Quantity

  mol-mole-amount of substance
• Symbol-Name-Quantity

  cd-candela-luminous intensity
• The second, symbol s, is the SI unit of time. It is defined by taking the fixed numerical value of the caesium frequency, ∆νCs, the unperturbed ground-state hyperfine transition frequency of the caesium 133 atom, to be 9 192 631 770 when expressed in the unit Hz, which is equal to s ^-1
• The metre, symbol m, is the SI unit of length. It is defined by taking the fixed numerical value of the speed of light in vacuum, c, to be 299 792 458 when expressed in the unit m s ^-1 , where the second is defined in terms of the caesium frequency ∆νCs.
• The kilogram, symbol kg, is the SI unit of mass. It is defined by taking thenfixed numerical value of the Planck constant, h, to be 6.626 070 15 x 10^-34 when expressed in the unit J s, which is equal to kg m^2 s^-1, where the metre and the second are defined in terms of c and ∆νCs.
• A one-significant-figure estimate or calculation is called an

  order-of-magnitude estimate
• Dimensional analysis can be used to estimate an answer or guess a formula even if you know almost nothing about the physics involved Steps:

  1. Think about what the desired quantity might depend on
  2. Assume a power-law formula
  3. Require unit consistency
  4. Assume that any unitless constant is approximately 1