1.5 Forces

• Force


  A push or a pull in a specific direction. It is a vector quantity.
• Force


  • Unit: newton, N
• Effects of a force

  • Change shape/appearance
  • Change size/volume
  • Change motion/velocity
• Types of forces


  • Weight
  • Friction
  • Drag (for liquid and gas)
• Forces produce heat
• Hooke's law

  The extension of a spring is directly proportional to the force applied provided that the limit of proportionality is not exceeded.
• Hooke's law

  • force
  • spring constant "stiffness"
  • extension
• Resultant force


  The single equivalent force when multiple forces are combined.
• Calculating resultant force


  1. Same direction ---> add

  2. Opposite direction ---> subtract

  
• Parallelogram of forces


  Pretend the forces act one at a time. Rearrange the arrows such that they form a parallelogram.
• Resultant force


  If an object has a resultant force, it will accelerate (or decelerate) in the same direction as the force.

  
• Centripetal force


  The resultant force that acts towards the centre of a circular motion.

  
• Centripetal force


  • If speed increases, force increases (mass and radius constant)
  • If radius increases, force decreases (mass and speed constant)
  • If mass increases, force increases to keep speed and radius constant.

  
• Racing line uses large radius to maintain high speed when cornering
• "Ring of death" needs high speed to have enough centripetal force to 'stay' on the wall
• Moment


  Moment = force × perpendicular distance

  
• • Not perpendicular: less moment
  • Perpendicular: maximum moment
  • Parallel: no moment
• Equilibrium


  A system is in equilibrium when both resultant force and resultant moment are zero.
• Finding the centre of mass of a flat object

  1. Hang object from a point near the edge.

  2. Hang plumb line from the same point.

  3. Draw line along plumb line.

  4. Repeat 1-3 from a new point.

  5. Intersection is the centre of mass.
• Centre of gravity


  The point in the body or system where the mass appears to be concentrated or where gravity appears to act.

  
• Depending on the shape, the centre of gravity may be inside or outside the object.
• Stability


  • An object is stable when the centre of gravity is within its base.
  • It is more stable with a wider base and a lower centre of gravity.