forces

• Contact and Non-Contact Forces


  • Contact forces are where the objects have to be touching for the force to act, e.g. friction, air resistance, tension in ropes, normal contact force
  • Non-contact forces are where the objects do not need to be touching for the force to act, e.g. magnetic force, gravitational force, electrostatic force
• Vectors


  • Have magnitude and direction
• Vector quantities


  • Force
  • Velocity
  • Displacement
  • Acceleration
  • Momentum
• Scalar quantities


  • Speed
  • Distance
  • Mass
  • Temperature
  • Time
• Velocity


  A vector quantity, but speed is a scalar quantity
• When two objects interact

  There is a force produced on both objects
• Interaction pair


  A pair of forces that are equal and opposite and act on two interacting objects (Newton's Third Law)
• Non-contact force


  • The gravitational force between the Sun and the Earth
• The Sun is attracted to the Earth


  The Earth is attracted to the Sun
• A chair rests on the ground

  The ground pushes back at the chair with the same force (the normal contact force)
• Gravitational Force


  Force of Attraction Between Masses
• Gravitational Field


  Causes all things to fall towards the ground
• Weight


  Force acting on an object due to gravity, the pull of the gravitational force
• Mass


  Property of an object, does not change with location
• Weight and Mass


  Not the same, weight depends on gravitational field strength
• Weight of an object depends on the strength of the gravitational field at the location of the object</b>
• Weight is measured in newtons, mass is measured in kilograms
• Weight and mass are directly proportional
• Calculating weight


  Weight (N) = Mass (kg) x Gravitational Field Strength (N/kg)
• Gravitational field strength


  • 9.8 N/kg on Earth, 1.6 N/kg on the Moon
• Increasing the mass of an object increases its weight
• Resultant Force


  Overall force on a point or object
• Free Body Diagram


  • Shows all the forces acting on an object
• Finding resultant force using scale diagram

  Draw forces tip-to-tail, the line from start of first to end of last is the resultant force
• If a resultant force moves an object, work is done
• Work Done


  Energy transferred when a force moves an object through a distance
• Work done is measured in joules (J)
• Equilibrium


  When the forces on an object are balanced, resulting in a zero resultant force
• Splitting a force into components


  Draw the force on a grid and construct horizontal and vertical components
• Elastic Deformation


  When an object can return to its original shape after a force is removed
• Inelastic Deformation


  When an object does not return to its original shape after a force is removed
• Hooke's Law


  Extension is directly proportional to force, F=ke
• Spring Constant


  Measure of the stiffness of a spring, in N/m
• There is a limit of proportionality where Hooke's Law stops applying
• Spring constant


  Measure of the stiffness of a spring, the greater the spring constant the stiffer the spring
• The equation for Hooke's law also works for compression
• There is a limit to the amount of force you can apply to an object for the extension to keep on increasing proportionally
• Graph of force against extension for an elastic object


  • There is a maximum force above which the graph curves, showing that extension is no longer proportional to force
  • This is known as the limit of proportionality and is shown on the graph at the point marked P
• The graph can also be extension-force, with the same features
• Spring constant


  Measured in N/m