Topic 8 + 9 - Forces

• Energy is never created or destroyed, it is only transferred
• When a system changes, energy is transferred from one form to another
• Electrical energy changing into kinetic energy


  • in a motor
• Closed system

  A system which experiences no external forces
• Closed systems


  • Circuit
  • Skydiver
• In a closed system, the total energy remains the same at all times
• Change in gravitational potential energy


  mass × gravitational field strength × change in vertical height
• Kinetic energy

  ½ × mass × (speed)2
• Work done

  This is when an external force transfers energy to a system
• Work done examples


  • A human doing work to push a ball up a hill
• Electric

  Chemical potential energy (stored in batteries) forms electric
• Heating

  Electric transfers to thermal energy, as work is done against the heating coils causing it to heat up
• Work done

  force × distance moved in the direction of the force
• Energy is rarely 100% transferred into the desired form
• Some energy is dissipated so it is stored in less useful ways
• Mechanical processes are wasteful when they get hot (usually due to friction)
• Energy is dissipated as heat, causing a rise in temperature
• The energy lost to the surroundings is due to wasteful processes
• Power

  The rate at which energy is transferred
• Power

  energy transferred or work done ÷ time taken
• 1 Watt is equal to 1 Joule per second
• Efficiency

  Measure of how much energy is not lost as waste
• Efficiency

  useful output energy ÷ total input energy
• Interaction of objects at a distance without contact

  • Gravitational attraction: There is an attraction between two objects with mass, the larger mass gives greater attraction
  • Electrostatic attraction/repulsion: A larger charge gives greater force, like charges repel, opposite charges attract
  • Magnetic attraction/repulsion: A stronger magnet gives a stronger field, having a greater force, like poles repel, opposite poles attract
• Interaction of objects with contact

  • Normal contact force: The force is perpendicular to the plane of contact
  • Friction: Surfaces that are rough cause friction when moved
• Vector

  Has size and direction e.g. a force of 10N directed downwards
• Scalar

  Has just size, so direction plays no part in describing the value
• Vector diagrams

  Shows the direction of forces that are present in a situation
• Free body diagrams

  Shows the direction of forces that are present in a situation
• Reaction force

  Always acts at the normal to the line of contact, from the point of contact
• Friction

  Acts in the opposite direction to movement, along the line of contact
• Weight

  Always acts downwards, acting from the Centre of Mass
• Scale Drawings

  The length of each arrow represents its size (in relation to the other forces acting on the object), so direction with larger arrows shows resultant force. If arrows are in opposite directions with equal length, the forces cancel out, so the object is in equilibrium and travels at a constant velocity
• Isolated solid systems mean no forces are present that come from a source outside the system
• Skydiver

  Forces that act are air resistance and weight. Initially, the skydiver has no air resistance and the only force acting on him is weight. As he falls, he accelerates, increasing his speed, making air resistance increase, resulting in a decrease in resultant force and acceleration. Eventually, weight and air resistance are equal and balanced, so there is no resultant force, and there is no acceleration
• Speed

  Makes air resistance increase
• Air resistance increase

  Resultant force decreases
• Resultant force decreases

  Acceleration decreases as F=ma, so not speeding up as quickly
• Weight and air resistance are equal and balanced

  No resultant force
• Vehicle

  Initially, low air resistance and thrust is only hindered by friction