Definitions (Standard Questions)

• SI base quantities and their units

  • electric current (ampere, A)
  • mass (kilogram, kg)
  • length (metre, m)
  • time (second, s)
  • thermodynamic temperature (kelvin, K)
  • amount of substance (mole, mol)
  • luminous intensity (candela, cd)
• Systematic error


  The reading is larger or smaller than the true reading and can't be eliminated by repeating and averaging. Due to an error in the measuring instrument.
• Random error


  Equal scatter in readings about the true reading and can be eliminated by averaging. Error is due to the observer.
• Precision


  Determined by the size of the random error (range) in the measurement. Normally the size of the smallest division on the measuring instrument.
• Accuracy


  The degree to which a measurement approaches the 'true value'.
• Uncertainty


  The total range of values within which the measurement is likely to lie.
• Scalar quantity


  A quantity which can be described fully by giving only its magnitude.
• Vector quantity


  A vector quantity has magnitude and direction.
• Scalar quantities


  • distance
  • speed
  • mass
  • pressure
  • temperature
  • energy
  • time
• Vector quantities


  • displacement
  • velocity
  • acceleration
  • force
  • weight
  • momentum
  • torque
  • electric/gravitational fields
• Displacement


  Distance travelled in a straight line in a specified direction from a point.
• Velocity


  Rate of change of displacement.
• Acceleration


  Rate of change of velocity.
• Experiment to determine the acceleration of free fall using a falling body


  1. Measure height s

  2. Drop object from rest

  3. Record time taken t

  4. Repeat t measurements and find mean

  5. Take several readings of s and t

  6. Plot t^2 against s and determine gradient

  7. Calculate acceleration of free fall g from g = 2/gradient
• Mass


  The property of a body which resists a change in motion.
• Weight


  The force due to a mass in a gravitational field.
• Linear momentum


  The product of mass and velocity.
• Force


  The rate of change of momentum.
• During a collision, the change in momentum of A

  Is equal and opposite to that of B
• Newton


  One newton is defined as the force which will give a 1kg mass an acceleration of 1ms^-2 in the direction of the force.
• Centre of gravity


  The single point within a body at which the entire weight of the body may be considered to act.
• Moment of a force


  The product of the force and the perpendicular distance of the line of action of the force from the pivot.
• Torque of a couple


  The product of one of the forces and the perpendicular distance between the forces.
• Equilibrium


  Sum of forces in any direction is zero or no resultant force, and sum of moments about any point is zero or no resultant moment.
• Drawing a force vector triangle in equilibrium

  Each force vector is represented by an arrow showing the magnitude and direction. The arrows are joined head to tail in a closed loop.
• Density


  Density of a substance is defined as its mass per unit volume.
• Pressure


  Force per unit area, where the force acts perpendicularly to the area.
• Work done


  The force multiplied by the displacement in the direction of the force.
• Joule


  When a force of one newton moves its point of application by one metre in the direction of the force, one joule of work is done.
• Power


  Work done per unit time.
• Watt


  Rate of transfer of energy of one joule per second.
• Joule


  Unit of work
• Energy cannot be created or destroyed. It can only be converted from one form to another.
• Power


  Work done per unit time
• Watt


  Rate of transfer of energy of one joule per second
• Potential energy


  Potential (stored) energy is the ability of an object to do work as a result of its position or shape
• Gravitational potential energy


  Energy possessed by a mass due to its position in a gravitational field
• Kinetic energy


  Energy of a mass due to motion
• Hooke's law


  Provided the limit of proportionality is not exceeded, the extension of a body is proportional to the applied load
• Tensile stress


  The force per unit area applied on an object normal to the surface