Forces

• Vector


  A quantity that has magnitude and direction
• Scalar


  A quantity that has just magnitude
• Generally, scalars cannot be negative, but vectors can be, as a certain direction is positive
• Scalar quantities


  • Speed
  • Distance
  • Time
  • Mass
  • Energy
• Vector quantities


  • Velocity
  • Displacement
  • Acceleration
  • Force
  • Momentum
• Speed is only velocity when given a direction
• Imagine a ball thrown off a cliff, displacement is 0 at height of cliff
• Above the cliff the ball has positive displacement, and below the clifftop the ball has negative displacement
• A force is a push or pull that acts on an object due to the interaction with another object
• Types of forces


  • Non-Contact
  • Contact
• Non-Contact forces


  Forces between objects that are physically separated
• Non-Contact forces


  • Electrostatic
  • Gravitational attraction
• Contact forces


  Forces between objects that are physically touching
• Contact forces


  • Normal contact force
  • Friction
• All matter has a gravitational field, and attracts all other matter
• Weight


  The force exerted on a mass by the gravitational field
• Weight = mass × gravitational field strength
• Weight, W, is in newtons, N and mass, m, is in kilograms, kg
• Weight is measured by a force meter (calibrated spring-balance)
• On earth, g = 9.8
• The gravitational field strength, g, at two different planets will be different
• Acceleration in free fall is due to gravity, and is the same as g, i.e. 10 m/s²
• The weight of an object is considered to act at the object’s centre of mass
• Resultant Force


  A single force representing the sum of all the forces acting on an object
• If more than one force acts along a straight line, the resultant can be found by adding or subtracting them
• Forces that act on a skydiver are air resistance and weight
• Initially, the skydiver has no air resistance and the only force acting on him is weight
• As the skydiver falls, he accelerates, increasing his speed
• As air resistance increases, the resultant force from weight decreases
• Eventually, when air resistance equals weight, there is no resultant force
• When the resultant force is 0, they travel at terminal velocity
• Free Body Diagrams


  Show the forces (and their directions) acting on an object

  
• Work Done


  Force × Distance
• Work Done is measured in joules, J
• Work Done is when energy is transferred from the object doing the work to another form
• One joule of work is done when a force of one newton causes a displacement of one metre
• Elastic Deformation


  The object returns to its original shape when the load has been removed

  
• Plastic Deformation


  The object does not return to its original shape when the load has been removed
• Hooke’s Law


  The extension of an elastic object is directly proportional to the force applied, provided that the limit of proportionality is not exceeded

  
• Moment of a Force = force × perpendicular distance