Forces

• Scalar quantity


  Quantity that has magnitude (size) only, and no direction
• Vector quantity


  Quantity that has both magnitude (size) and direction
• Representing vectors


  • Using an arrow, where the length represents the magnitude and the direction represents the direction of the vector
• Force


  A push or pull that acts on an object due to the interaction with another object
• Force


  • Has magnitude (size) and direction
  • Is a vector quantity
  • Unit is the Newton
• Types of forces


  • Contact forces
  • Non-contact forces
• Contact force


  Force that occurs when two objects are physically touching
• Contact forces


  • Tension in a rope
  • Friction (e.g. between airplane and water)
  • Air resistance (e.g. on a skydiver)
  • Normal contact force (e.g. between a lump and a table)
• Non-contact force


  Force that occurs when two objects are physically separated
• Non-contact forces


  • Gravitational force (e.g. between Earth and International Space Station)
  • Electrostatic force (e.g. between charged objects)
  • Magnetic force (e.g. in a magnetic field)
• Weight


  The force acting on an object due to gravity
• Calculating weight


  Mass in kilograms x Gravitational field strength in Newtons per kilogram
• Mass of an object


  Weight of the object is directly proportional
• Gravitational field strength


  A measure of the force of gravity in a particular location
• Gravitational field strength


  Depends on the location
• Center of mass


  The point at which the weight of an object can be considered to act
• Work done


  1. Force applied

  2. Object moves a distance

  3. Energy transferred
• Work done


  Energy transferred when a force moves an object
• The distance must be in the line of action of the force
• Work is measured in Joules (J)
• 1 Newton meter (Nm) of work = 1 Joule (J)
• Braking a car


  • Kinetic energy of car transferred to thermal energy in brakes
  • Car slows down as it loses kinetic energy
• Only the distance in the line of action of the force is relevant for work done
• Elastic deformation


  Changes in an object's length or shape that are reversible when the forces are removed
• Inelastic deformation


  Changes in an object's length or shape that are not reversible when the forces are removed
• Applying forces to an elastic material

  1. Stretching forces are equal in magnitude but opposite in direction

  2. Squeezing forces cause compression

  3. Applying multiple forces causes bending
• Elastic materials return to their original length or shape when forces are removed
• To change an object's length or shape, more than one force must be applied
• Force to stretch an elastic object

  Force (N) = Spring constant (N/m) x Extension (m)
• Elastic potential energy


  Stored in an object when it is stretched or compressed
• Work done = Elastic potential energy (if object is not inelastically deformed)
• Moment


  The turning effect of a force
• Calculating the size of a moment

  1. Force in newtons

  2. Distance in meters

  3. Multiply force and distance
• The unit for moment is newton meters (N·m)
• The distance used to calculate moment must be the perpendicular distance from the line of action of the force to the pivot
• Moment


  A turning effect of a force, calculated in Newton meters by multiplying the force in Newtons by the distance in meters
• The distance must be perpendicular from the pivot to the line of action of the force
• Balanced moments


  The clockwise moment equals the anti-clockwise moment
• Calculating balanced moments


  Force x Distance (clockwise) = Force x Distance (anti-clockwise)
• Calculating unknown counterweight force in balanced moments

  Anti-clockwise moment / Distance of counterweight = Force of counterweight