Chapter 2 Book Notes (pg. 24-33)

Standard Units of Measurement

• physics is the study of interactions of matter and energy in all their diverse forms

• only three measurable quantities are considered basic .. these base quantities are length, mass, and time and they are the building blocks of all other quantities

• the secondary quantities are called derived quantities because they are derived from a combination of one or more of the three base quantities EX: volume is length cubed, mass density is mass divided by volume, velocity Is length divided by time

• additional quantities are designed to support measurement in specialized areas of science and technology … these additional quantities are called special quantities

• special quantities are those of exposure, dose, effective dose, and radioactivity

• for many years the standard unit of length was accepted to be distance between two lines engraved on. platinum-iridium bar kept at the International Bureau of Weights and Measures in Paris, France

• in 1960 the need for a more accurate standard of length led to redefinition of the meter in terms of the wavelength or orange light emitted from an isotope of krypton

• one meter is now defined as the distance traveled by light in 1/299,792,468 second

• the kilogram was originally denied to be. the mass of 1000cm³ of water at 4 degrees C

• the kilogram is a unit of mass and the newton and the pound, a British unit, are units of weight

• the standard unit of time is seconds(s)

• originally the second was defined in terms of the rotation of the earth on its axis. In 1956 it was redefined as a certain fraction of. the tropical year 1900

• in 1964 the need for a better standard of time led to another redefinition

• every measurement has two parts a magnitude and a unit

Mechanics

• mechanics is a segment of physics that deals with objects at rest and objects in motion. objects at rest are static. objects in motion are dynamic

• the motion of an object can be described with the use of two terms velocity and acceleration

• velocity, sometimes called speed, is a measure of how fast something is moving or, more precisely, the rate of change of its position with time

• The rate of change in velocity with time is acceleration. How “quickly or slowly” the velocity is changing

• if velocity is constant, acceleration is zero

• newtons second law is a definition of the concept of force

• force can be thought of as a push or pull on an object

• newtons third law of motion states that for every action, there is an equal and opposite reaction

• action was newtons word for force

• if you push a heavy block, the block will push back on you with the same force you apply

• weight is a force on a body caused by the pull of gravity on it

• weightlessness observed in outer space is attributed to the absence of gravity thus the value of gravity in outer space is zero

• the weight of an object can vary according to the value of gravity acting on it .. however note that. the mass of an object does not change

• the product of the mass of an object and its velocity is called momentum

• the greater the velocity of an object, the more momentum the object possesses

• the total momentum before any interaction is equal to the total momentum after the interaction

• the work done on an object is the force applied to that object times the distance over which it Is applied

• there are many forms of energy

• the law of conservation of energy states that energy may be transformed from one form to another, but it cannot be created or destroyed

• the total amount of energy is constant

• two forms of energy are often used in radiological sciences .. kinetic and potential energy

• kinetic energy is energy in motion

• potential energy is the stored energy of position or configuration (has the potential to move) EX: a textbook on a desk has potential energy because of its height above the floor.. it has the ability to do work by falling to the ground

• heat is a form of energy that is very important to radiologic technologists .. excessive heat a deadly enemy of an x-ray tube can cause permanent damage

• the more rapid. anddisordered the motion of molecules, the more heat an object contains

• the unit of heat is calorie .. defined as the heat necessary to raise the temperature of 1g of water by 1 degree C

• conduction is the transfer of heat through a material or by touching

• molecular motion from a high temperature object that touches a lower temperature object equalizes the temperature of both

• conduction is easily observed when a hot object and a cold object are placed in contrast

• heat is conducted from an xray tube anode through the rotor to the insulating oil

• convection is the mechanical transfer of “hot” molecules in. agas or liquid from one place to another

• thermal radiation is the transfer of heat by the emission of infrared radiation .. the reddish glow emitted by hot objects is evidence of heat transfer by radiation

• an xray tube cools primarily by radiation

• the relationship between temperature and energy is often represented by an energy thermometer

• we consider xrays to be energetic although on the cosmic scale, they are rather ordinary