Topic 9 – Forces and their effects

• gravitational attraction - there is attraction between two objects with mass, the larger mass gives greater attraction
• electrostatic attraction/repulsion - a larger charge gives a greater force, like charges repel, opposites attract
• magnetic attraction - a stronger magnet gives a stringer field, having a greater force. like poles repel. opposite poles attract
• normal contact force - the force is perpendicular to the plane fo contact
• friction - surfaces that are rough cause friction when moved
• a vector has a size and a direction

  force, velocity, weight, displacement, acceleration, momentum
• scalar has just size

  mass, distance, speed, temperature, energy, density
• free body diagrams

  shows the direction of forces that are present in a situation

  the reaction force always acts to the normal

  friction acts in the opposite direction to movement

  weight always acts down, from the centre of mass

  
• scale drawings

  the length of each arrow represents its size

  so direction with larger arrows shows resultant force

  • if arrows are equal in opposite directions, they are equal in magnitude but opposite in direction, forces are cancelled out, in equilibrium, so travels at a constant velocity.

  
• rotation -

  • if an object is attached to a pivot point, a point where it can rotate but not move away from
  • and a force is applied not towards the point, the object will not rotate but will just be held still, as there is no resultant force
  • if the force is applied perpendicular to the object, it will move around the pivot
  • if the force is applied not perpendicular, you need to find the perpendicular distance to see what direction it will turn

  
• moment = Fd

  moment of a force (newton metre, Nm) = force (newton, N) x distance perpendicular to the direction of force (metre, m)
• equilibrium occurs when - sum of anticlockwise moments = sum of clockwise moments
• gears can change speed, force or direction by rotation
• if a gear is connected to another gear with fewer teeth, the second gear will turn faster but with less force in an opposite direction to the first gear
• if a gear is connected to a gear with more teeth, it will turn slower with more force in an opposite direction
• the second gear will always turn in the opposite direction
• lubrication - reduces friction, so reduces unwanted energy transfer (so less hear loss etc) and increases efficiency