topic 9 - forces

• Examples of interaction of objects at a distance without contact:

  • Gravitational attraction: larger mass gives greater attraction
  • Electrostatic attraction/repulsion: larger charge gives greater force, like charges repel, opposite charges attract
  • Magnetic attraction/repulsion: stronger magnet gives stronger field, like poles repel, opposite poles attract
• Examples of interaction of objects with contact:

  • Normal contact force: perpendicular to the plane of contact
  • Friction: surfaces that are rough cause friction when moved
• A vector has size and direction, e.g., a force of 10N directed downwards; examples include weight, velocity, force, displacement
• Scalar quantities have only size, direction plays no part in describing the value, e.g., mass, distance, speed
• Free body diagrams show the direction of forces present in a situation
• Points to note in free body diagrams:

  • Reaction force acts normal to the line of contact, from the point of contact
  • Friction acts opposite to movement, along the line of contact
  • Weight always acts downwards from the center of mass
• Scale drawings: the length of each arrow represents its size in relation to other forces acting on the object; direction with larger arrows shows resultant force
• In isolated solid systems, no forces are present from an external source outside the system
• Forces acting on a skydiver:

  • Initially, only weight acts
  • As the skydiver falls, air resistance increases, reducing resultant force and acceleration
  • Eventually, weight and air resistance balance, reaching terminal velocity with no acceleration
• Forces acting on a vehicle:

  • Initially, low air resistance, thrust hindered by friction
  • Air resistance increases, decreasing resultant force
  • At terminal velocity, thrust is balanced by drag and friction, resulting in no net force acting
• Rotation occurs when an object is attached to a pivot point and a force is applied not towards the point; moment of a force = force × distance perpendicular to the direction of the force
• Equilibrium occurs when the sum of anticlockwise moments equals the sum of clockwise moments
• Gears can change speed, force, or direction by rotation:

  • If connected to a gear with fewer teeth, the second gear turns faster but with less force in the opposite direction
  • If connected to a gear with more teeth, the second gear turns slower with more force in the opposite direction