Topic 9 ~ Forces and Their Effects

• Examples of interaction of objects at a distance without contact

  • Gravitational attraction
  • Electrostatic attraction/repulsion
  • Magnetic attraction/repulsion
• 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 stronger field, having a greater force
  • Like poles repel, opposite poles attract
• Examples of interaction of objects with contact

  • Normal contact force
  • Friction
• 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
• Examples of vectors


  • Weight
  • Velocity
  • Force
  • Displacement
• Scalar


  Has just size - so direction plays no part in describing the value
• Examples of scalars


  • Mass
  • Distance
  • Speed
• Free body diagrams


  • Shows the direction of forces that are present in a situation
  • The 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 line of contact
  • Weight always acts downwards, acting from Centre of Mass
• Scale drawings


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


  1. Initially, the skydiver has no air resistance and the only force acting on him is weight

  2. As he falls, he accelerates, increasing his speed

  3. This makes air resistance increase

  4. Therefore, the resultant force decreases

  5. Eventually weight and air resistance are equal and balanced, so there is no resultant force and terminal velocity is reached
• Vehicle


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

  2. Air resistance increases, decreasing resultant force

  3. Eventually the car is travelling at terminal velocity, where the thrust is balanced by drag and friction, so no resultant force acts
• Moment of a force

  Force (newton, N) × distance perpendicular to the direction of the force (metre, m)
• Equilibrium occurs when: sum of anticlockwise moments = sum of clockwise moments
• Gears


  • Can change speed, force or direction by rotation
  • If connected to a gear with fewer teeth, the second gear will turn faster but with less force in opposite direction
  • If connected to a gear with more teeth, the second gear will turn slower but with more force in opposite direction
  • The second gear will always turn in the opposite direction
• Lubrication reduces friction, so reduces unwanted energy transfer (so less heat loss etc.) and increases efficiency