Unit 2

• Science is used to describe, explain and quantify information
• Physics


  The study of the interactions of matter & energy
• Radiation Physics


  The production of x-rays involves all the laws of physics and involves interactions between: Electrons inside the x-ray tube, Radiation and body tissues, X-rays and the image receptor
• Quantity


  What is being measured (e.g. length)
• Unit


  Value that describes the quantity measured (e.g. meter)
• Symbol


  Abbreviation of the unit (e.g. l)
• Three basic measurable quantities

  • Mass
  • Length
  • Time
• Mass


  The quantity of matter contained in any physical object
• Kilogram


  The standard unit of mass, represented by a platinum-iridium cylinder, originally defined as the mass of 1000 cm3 of water at 4 Celsius
• Length


  Measure of distance or dimension
• Meter


  The standard unit of length, defined as the distance traveled by light in 1/299 792 458 of a second
• Time


  The progression of events from the past to the present into the future
• Second


  The standard unit of time, measured by an atomic clock based on the vibration of cesium atoms
• Measurable Quantities and their SI Units

  • Mass - Kilogram (kg)
  • Length - Meter (m)
  • Time - Second (s)
• Derived Quantities


  Called secondary quantities, they are derived from a combination of one or more of the base units, e.g. Volume, Velocity, Mass Density
• Special Quantities


  • Radiologic Units: Effective Dose measured in Sieverts (Sv)
  • Computer Memory: Measured in kilobyte (KB), megabyte (MB), and gigabyte (GB)
• Every measurement has 2 parts: Magnitude & Unit
• Measurements must be well-defined, standardized, and reproducible
• International agencies redefine standards as science and technology progresses
• Système International d'Unités (SI)

  The international standard for measurements, also known as the metric system, established in 1960 with seven base units
• Mechanics


  Physics that deal with objects at rest (static) and objects in motion (dynamic)
• Velocity


  Measure of how fast an object is moving, rate of change of position with time, also called speed
• Meters per second (m/s)


  The SI unit of velocity
• Speed of Light (c)


  The speed at which light and all other electromagnetic radiation travel in a vacuum, a constant at 3 x 10^8 m/s or 186,000 miles per second
• Acceleration


  Rate of change of velocity with time, how quickly or slowly velocity changes
• Meters per second squared (m/s^2)


  The SI unit of acceleration
• Newton's Laws of Motion

  • 1st Law: Inertia - A body at rest will remain at rest & a body in motion will continue in motion until acted on by an external force
  • 2nd Law: Force - The force that acts on an object is equal to the mass of the object multiplied by the acceleration produced
  • 3rd Law: Action/Reaction - For every action there is an equal and opposite reaction
• Weight


  The force on a body caused by gravity's influence, SI unit is newton (N)
• Momentum


  The product of a mass of an object and its velocity, force is needed to stop momentum
• Work


  The work done on an object is the force applied to that object times the distance over which that force is applied, SI unit is joule (J)
• Power


  The rate of doing work, SI unit is watt (W), British unit is horsepower (hp)
• Energy


  The ability to do work, exists in two forms: kinetic energy and potential energy
• Heat


  Kinetic energy of the random motion of molecules, transferred by conduction, convection, and radiation
• 1. ray tubes cool primarily by radiation, with conduction and convection as secondary methods
• Temperature


  Measurement of hotness or coldness, usually calibrated with two reference points: the freezing and boiling points of water, measured in Celsius (C), Fahrenheit (F), and Kelvin (K)