Forces

• What is a scalar quantity 


  Scalar quantities have magnitude only.
• Vector quantities have magnitude and an associated direction. A vector quantity may be represented by an arrow. The length of the arrow represents the magnitude, and the direction of the arrow . the direction of the vector quantity.
• A force is a push or pull that acts on an object due to the interaction with another object.
• contact forces – the objects are physically touching • non-contact forces – the objects are physically separated
• Examples of contact forces include friction, air resistance, tension and normal contact force.
• Examples of non-contact forces are gravitational force, electrostatic force and magnetic force.
• What type of quantity is force

  Vector quantity
• What is weight 


  Weight is the force acting on an object due to gravity
• The force of gravity close to the Earth is due to the gravitational field around the Earth. The weight of an object depends on the gravitational field strength at the point where the object is.
• The force of gravity close to the Earth is due to the gravitational field around the Earth. The weight of an object depends on the gravitational field strength at the point where the object is.
• How is weight measured

  In newtons with a calibrated spring balance or a newtonmeter
• How is Mass measured

  In kg
• How is gravitational field strength measured in 


  N/kg
• The weight of an object may be considered to act at a single point referred to as the object’s ‘centre of mass’. The weight of an object and the mass of an object are directly proportional.
• Resultant force is a single force that can replace a number of forces acting on an object
• Describe work done


  Work done on an object is when a force causes an object to move through a distance causing displacement on it
• How is work done measured

  In joules
• How is distance measured

  In metres
• One joule of work is done when a force of one newton causes a displacement of one metre. 1 joule = 1 newton-metre
• Work done against the frictional forces acting on an object causes a rise in the temperature of the object
• Elastic deformation


  Changes in an object's length or shape that are reversible when the forces are removed
• Inelastic deformation


  Changes in an object's length or shape that are not reversible when the forces are removed
• Applying forces to an elastic material


  1. Stretching forces are equal in magnitude but opposite in direction

  2. Squeezing forces cause compression

  3. Applying multiple forces causes bending
• Elastic materials return to their original length or shape when forces are removed
• To change an object's length or shape, more than one force must be applied
• Force to stretch an elastic object

  Force (N) = Spring constant (N/m) x Extension (m)
• Elastic potential energy


  Stored in an object when it is stretched or compressed
• Work done = Elastic potential energy (if object is not inelastically deformed)
• Investigating the relationship between force and extension for a spring

  1. Set up clamp stand, bosses, and clamps

  2. Place heavy weight on clamp stand

  3. Attach meter rule under spring, with top at 0 point

  4. Attach wooden splint pointer to bottom of spring, keep horizontal

  5. Read unstretched length of spring

  6. Add 1 Newton weight, read new pointer position

  7. Continue adding 1 Newton weights, read pointer positions

  8. Calculate extension for each weight by subtracting unstretched length

  9. Plot extension vs weight graph
• Graph of extension vs weight

  • Straight line through origin
  • Linear relationship between force and extension
  • Elastic deformation - extension returns to 0 when weight removed
  • Inelastic deformation - over-stretching, extension remains when weight removed
• Linear relationship


  Straight line graph, extension directly proportional to weight
• Nonlinear relationship


  Curved graph, extension not directly proportional to weight
• Elastic deformation


  Extension returns to 0 when weight removed
• Inelastic deformation


  Extension remains when weight removed, spring over-stretched
• Limit of proportionality


  Point on graph where relationship becomes nonlinear
• Spring constant is the same for any part of the graph as long as the limit of proportionality is not exceeded
• What is distance 


  Distance is how far an object moves. Distance does not involve direction. Distance is a scalar quantity
• What is displacement 


  Displacement includes both the distance an object moves, measured in a straight line from the start point to the finish point and the direction of that straight line. Displacement is a vector quantity.
• Describe speed 


  Speed does not involve direction. Speed is a scalar quantity. The speed of a moving object is rarely constant. When people walk, run or travel in a car their speed is constantly changing
• Factors of speed 


  age, terrain, fitness and distance travelled