Topic 5- forces

• Vector


  Quantity with both magnitude and direction
• Scalar


  Quantity with only magnitude
• Examples of scalars


  • Speed
  • Distance
  • Time
  • Energy
  • Mass
• Examples of vectors


  • Velocity
  • Displacement
  • Acceleration
  • Force
  • Momentum
• Scalars cannot be negative, but vectors can be, as a certain direction is positive
• Speed is only velocity when given a direction
• A car travelling round a roundabout at constant speed

  Its velocity is constantly changing therefore it is accelerating
• Force


  A push or pull that acts on an object due to the interaction with another object
• Types of forces


  • Non-contact (electrostatic, gravitational attraction)
  • Contact (normal contact force, friction)
• Gravitational field


  All matter has a gravitational field, and attracts all other matter
• Weight


  The force exerted on a mass by the gravitational field, in Newtons
• On Earth, g = 9.8
• The weight of an object is considered to act at the object's centre of mass
• Resultant force


  A single force representing the sum of all the forces acting on an object
• Skydiver example


  1. Initially no air resistance, only weight force

  2. As he falls, air resistance increases, resultant force decreases

  3. Eventually air resistance and weight balance, no resultant force, terminal velocity
• Free body diagram


  Shows the forces (and their directions) acting on an object
• Resolving forces


  A force F at angle θ to the ground can be resolved parallel and perpendicular to the ground using Pythagoras' Rule

  
• Work done


  Energy transferred from the object doing the work to another form
• Work done against frictional forces causes a rise in temperature of the object
• Deformation


  Changing the shape of an object
• Types of deformation


  • Elastic (object returns to original shape)
  • Plastic (object does not return to original shape)
• Hooke's law


  The extension of an elastic object is directly proportional to the force applied, provided the limit of proportionality is not exceeded

  
• Force/extension graph


  • Linear in elastic region following Hooke's law, gradient is k
  • Non-linear in plastic region, not following Hooke's law

  
• Moment of a force

  Force x perpendicular distance from pivot
• Equilibrium is when the sum of anticlockwise moments = sum of clockwise moments
• Gears


  Can change speed, force or direction by rotation
• Gear system


  Smaller gear turns faster with less force, larger gear turns slower with more force, both turn in opposite direction
• The second gear will always turn in the opposite direction to the first gear
• Pressure


  Force per unit area
• Pressure produces a net force at right angles to any surface
• Buoyancy force


  Upwards force that counteracts the weight of a floating object, equal to the weight of the fluid displaced
• The atmosphere gets less dense with increasing altitude
• The weight of the air causes the pressure in the atmosphere
• Upthrust


  A partially (or totally) submerged object experiences a greater pressure on the bottom surface than on the top surface, creating a resultant force upwards
• Earth's Atmosphere


  • A thin layer (relative to size of the earth) of air around the Earth
  • The atmosphere gets less dense with increasing altitude
• The atmosphere is a thin layer (relative to the size of the Earth) of air round the Earth. The atmosphere gets less dense with increasing altitude.
• Idealised Assumptions, for a simple model of the atmosphere

  • Isothermal, so it is all at the same temperature
  • Transparent to solar radiation
  • Opaque to terrestrial radiation
• Distance


  How far an object moves, does not involve direction, a scalar quantity
• 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, a vector quantity
• Speed


  Does not involve direction, a scalar quantity