Forces

• Gravity


  All objects with mass produce a gravitational field
• Gravitational field


  • Strength is 9.8 N/kg
  • Strength on the Moon is 1.6 N/kg
• Weight


  The force exerted on an object due to gravity
• Mass of an object


  Proportional to the weight of the object
• The greater the mass, the greater the weight
• The greater the gravitational field strength, the greater the weight
• Work


  The energy transferred when a force causes an object to move
• Work done

  • Warming up hands by rubbing them together
• Work is also done when a force causes an object to move against frictional forces
• Braking

  1. Thinking distance

  2. Reaction time

  3. Braking distance
• Thinking distance


  The distance a vehicle travels during the time it takes for the driver to react and apply the brakes
• Braking distance


  The distance a vehicle travels after the driver has applied the brakes
• Stopping distance


  The total distance a vehicle travels from the moment the driver sees a hazard to the moment the vehicle comes to a complete stop
• Factors that can affect stopping distance

  • Driver impairment (drugs, alcohol, distractions)
  • Weather conditions
  • Poor road surface
  • Poor vehicle conditions (worn brakes/tyres)
• Stopping distance = Thinking distance + Braking distance
• Newton's First Law


  An object remains in the same state of motion unless acted upon by an external force
• Newton's First Law


  1. Object remains in same state of motion

  2. Forces acting on object

  3. Curved state of motion
• Newton's Second Law


  The acceleration of an object is proportional to the net force acting on it and inversely proportional to its mass
• Newton's Second Law


  1. Resultant force

  2. Mass

  3. Acceleration
• Resultant force is proportional to mass and acceleration
• As mass increases, acceleration decreases, and vice versa
• Inertial mass


  Measure of how difficult it is to change the velocity of an object
• Newton's Third Law


  Whenever two objects interact, they exert equal and opposite forces on each other
• Newton's Third Law


  • Car tyre on a road
  • Tyre pushes the road backwards
  • Road pushes the tyre forwards
• Change of Shape


  When a force acts on an object, the object changes shape by bending, stretching, or compressing
• Elastic deformation


  • The change in shape is reversed when the force is removed
• Inelastic deformation


  • The change in shape is not fully reversed when the force is removed
• Elastic deformation


  • Rubber band
• Inelastic deformation


  • Metal can
• Hooke's Law


  The extension of an object is directly proportional to the force applied, within the limit of proportionality
• Extension


  An object increases in length
• Compression


  An object decreases in length
• Spring constant


  The proportional relationship between the force and the extension
• Limit of proportionality


  The point beyond which Hooke's Law is no longer true, and the relationship between force and extension becomes non-linear
• The limit of proportionality is the furthest point a material can be stretched and still return to its original shape
• Once a material goes beyond the elastic limit, it becomes inelastic and has a permanent change in shape
• Within the limit of proportionality


  The extension of a force spring is directly proportional to the force
• Beyond the limit of proportionality


  The relationship between force and extension becomes non-linear
• Momentum


  The product of mass and velocity
• Momentum


  • Vector quantity
  • Units: kg m/s