Forces Density & Kinetic theory

• Force


  That which changes or tends to change the shape or state of rest or motion of a body
• Effects of forces


  • Change the size or shape of a body
  • Change the speed of a body
  • Stop or start a body
  • Change the direction of movement of a body
• Sometimes a force does not cause effects 1-4 because it is probably canceling or balancing the effect of another force
• Unit of force


  Newton (newton meter)
• Types of force


  • Contact forces (friction, push, pull, squeeze, twist, lift)
  • Forces acting at a distance (gravitational, magnetic, electrostatic)
• Mass


  Measure of the quantity of matter in a body
• Weight


  Measure of the force exerted by gravity on a body
• Mass is constant, weight varies with location (e.g. on Earth vs Moon)
• Mass is measured in kilograms, weight is measured in newtons
• Balanced forces


  Forces that cancel each other out, resulting in no change in movement
• Unbalanced forces


  Forces that do not cancel each other out, resulting in a change in movement
• If forces are unbalanced, the overall force is known as the resultant
• Unbalanced forces


  • 20N to the left, 5N to the right
  • 10N up, 5N down
• Centre of gravity


  The point where the weight of an object is treated as acting
• Stability


  • A body will topple when the vertical line through its centre of gravity falls outside its base
  • Objects with a high centre of gravity are less stable than those with a low centre of gravity
• Moment


  The turning effect of a force, calculated as force x perpendicular distance from the pivot
• The unit for a moment is the newton metre (Nm) or newton centimetre (Ncm)
• Calculating moments


  • 8N force acting 2m from pivot, moment = 8 x 2 = 16Nm
  • 100N force acting 1.5m from pivot, moment = 100 x 1.5 = 150Nm
• The principle of moments states that when a body is in equilibrium, the sum of the clockwise moments about any point equals the sum of the anticlockwise moments about the same point
• Lever


  A device which can turn about a pivot, where an effort force is used to overcome a load force
• Levers


  • Can be used as a force multiplier (effort < load) or a distance multiplier (effort > load)
  • The moment of the effort force = the moment of the load force
• Volume


  The amount of space an object occupies, measured in cubic units (cm³, m³, etc.)
• Calculating volume


  • Volume = length x breadth x height
  • Volume of a liquid measured in ml or cm³ using a measuring cylinder
• The volume of an irregularly shaped object can be measured by the displacement method
• volume = 89
• The Volume of a liquid
• Units to measure volume of a liquid

  • militres (m)
  • liters
• 2 cm-1 m
• 1 litre 2000m-1000 cm
• To find the volume of a liquid

  1. Pour it into a measuring cylinder

  2. Read the scale
• Measuring the volume of a liquid

  • Measure to the bottom of the meniscus (curved surface)
  • Ensure your eye is level with the bottom of the meniscus
• Liquid A volume in ml 22
• Liquid C volume in ml 104
• Liquid C volume in cm³ 44
• Measuring the Volume of Irregularly Shaped Objects by the Displacement Method

  1. Partly fill a measuring cylinder with water and note the volume (V1)

  2. Lower the object into the water and note the new volume (V2)

  3. Calculate V₂-V1
• A graph of Mass against Volume for each substance gives a straight line of best fit through the origin
• Mass is directly proportional to volume
• mass = constant x volume


  This constant is the density of the material concerned
• If two quantities are directly proportional then whatever one quantity is multiplied by the other quantity is multiplied by the same value
• Density


  The mass of unit volume of that material