Physics

• Clinical thermometer


  • Needs to be very sensitive and accurate for the doctor or nurse to make adequate decisions regarding the patient
  • Requires only a small range because the temperatures for which it is used are only between 35°C and 42°C
• Fundamental quantities and their SI base units

  • mass (kilogram, kg)
  • length (metre, m)
  • time (second, s)
  • current (ampere, A)
  • temperature (kelvin, K)
  • amount of substance (mole, mol)
  • luminous intensity (candela, cd)
• Other quantities are derived from these fundamental ones, and SI derived units are combinations of the base units
• Calculating force (F)


  1. F = ma

  2. N = kg m s-1
• Derived quantities and their SI units

  • force (N = kg m s-1)
  • work or energy (Nm = kg m2 s-1)
  • power (W = kg m2 s-3)
  • pressure (Pa = N m-2)
  • charge (C)
  • voltage (V = J C-1)
  • resistance (Ω = V A-1)
  • frequency (Hz = s-1)
• Standard form or index notation

  M x 10p, where M is the mantissa (a number in decimal form with only one non-zero digit before the decimal point) and p is an integer
• Prefixes


  • pico (p, x 10-12)
  • nano (n, x 10-9)
  • micro (μ, x 10-6)
  • milli (m, x 10-3)
  • no prefix (x 100)
  • kilo (k, x 103)
  • mega (M, x 106)
  • giga (G, x 109)
  • tera (T, x 1012)
• Density


  Mass per unit volume
• Measuring density


  1. Mass can be obtained using an electronic balance

  2. Volume can be obtained by pouring a liquid into a measuring cylinder, measuring dimensions of a regularly shaped solid, or immersing an irregularly shaped solid in a liquid
• Unit of density


  Usually expressed in kg m-3 or g cm-3
• Scalars and vectors


  • Scalars (quantities with only magnitude): mass, length, time, temperature, area, volume, speed, pressure, distance
  • Vectors (quantities with magnitude and direction): force, momentum, displacement, velocity, acceleration, work, energy, power, resistance, current
• Forces


  • Gravitational forces (attractive forces due to mass)
  • Magnetic forces (attractive or repulsive forces due to magnetic polarity)
  • Electrostatic forces (attractive or repulsive forces due to electric charge)
  • Nuclear forces (extremely strong attractive forces binding subatomic particles)
  • Elastic forces (restoring forces when a body is stretched or compressed)
  • Mechanical forces (pushes, pulls, normal reactions, friction)
• Mass


  Quantity of matter making up a body
• Weight


  Force of gravity on a body
• Weight


  Depends on the gravitational field strength acting on the body
• On Earth, the generally accepted value of the gravitational field strength is 10 N kg-1, equal to the acceleration due to gravity of 10 m s-2
• Weight = mass x gravitational field strength (W = mg)
• Coplanar forces in equilibrium

  • 1. The sum of the forces in any direction is equal to the sum of the forces in the opposite direction (translational equilibrium)
  • 2. The sum of the clockwise moments about any point is equal to the sum of the anticlockwise moments about that same point (rotational equilibrium)
• Tackling problems involving moments

  1. Sketch a diagram showing all the forces acting on the body in equilibrium

  2. Select a suitable point about which to take moments

  3. Use the rules of translational and rotational equilibrium to formulate equations and solve for unknown forces and distances
• Moment of a force about a point

  The product of the force and the perpendicular distance of its line of action from the point
• The SI unit of moment is N m. Other units may be used, such as N cm or kN m, depending on the size of the moment
• Work is the product of a force and the distance moved in the direction of the force. Its unit is therefore also the product N m. To indicate the difference between the work and moment, the unit joule (J) is assigned to work
• Centre of gravity of a body


  The point through which the resultant gravitational force on the body acts
• Determining the centre of gravity of an object


  Balance the rod on the edge of a fulcrum or suspend it horizontally from a string until it balances. The point of support is then the centre of gravity
• Factors affecting the stability of an object

  • Height of its centre of gravity
  • Width of its base
  • Its weight
• Stable equilibrium

  A body is in stable equilibrium if, when slightly displaced, its centre of gravity rises and a restoring moment is created that returns the body to its base
• Unstable equilibrium

  A body is in unstable equilibrium if, when slightly displaced, its centre of gravity falls and a toppling moment is created which removes the body from its base
• Neutral equilibrium


  A body is in neutral equilibrium if, when slightly displaced, the height of its centre of gravity is unchanged and the body remains at rest in its new position
• Proportional limit (P)


  The point beyond which any further increase in the load applied to a spring will produce an extension that is no longer proportional to the force
• Elastic limit (E)


  The point beyond which any further increase in the load applied to a spring will produce a permanent stretch
• For loads (forces) within the elastic limit, there is elastic deformation; if the load is removed the spring returns to its original size and shape. Beyond the elastic limit, there is plastic deformation; the material is permanently stretched
• Hooke's law


  The force applied to a spring is proportional to its extension
• Variables in kinematics

  • distance (scalar)
  • displacement (vector)
  • speed (scalar)
  • velocity (vector)
  • acceleration (vector)
• Displacement-time graph

  Velocity = change in displacement / change in time (gradient)
• Velocity-time graph

  • Acceleration = change in velocity / change in time (gradient)
  • Distance = area between graph line and time axis (all areas are positive)
  • Displacement = area between graph line and time axis (areas above time axis are positive; areas below time axis are negative)
• Aristotle's theory of motion was based on observations rather than experiments, and was eventually discredited and replaced by Newton's laws of motion
• Newton's 1st law

  A body continues in its state of rest or uniform motion in a straight line unless acted on by a resultant force
• Newton's 2nd law


  The rate of change of momentum of a body is proportional to the applied force and takes place in the direction of the force
• Newton's 3rd law

  If body A exerts a force on body B, then body B exerts an equal but oppositely directed force on body A
• Momentum


  The product of a body's mass and its velocity