Forces

• Force

  A push or pull that acts on an object due to its interaction with another object
• Newtons

  The unit used to measure force
• Forces

  • Have both magnitude (measured in newtons) and direction (e.g. to the right)
• Vector quantities

  Quantities that have both magnitude and direction
• Types of forces

  • Contact forces
  • Non-contact forces
• Contact forces

  • Require the two objects to be physically touching
• Normal contact force

  Also known as the reaction force, it is the force exerted upwards by a surface to counteract the weight of an object on it
• Non-contact forces

  • Do not require the two objects to be touching
• Types of non-contact forces

  • Gravitational force
  • Magnetic force
  • Electrostatic force
• Fields of influence

  The surrounding area in which non-contact forces can act on objects
• As the distance between objects increases

  The strength of non-contact forces decreases
• Scalar quantities

  Physical quantities that only have a magnitude but no direction
• Magnitude

  Size, can be measured with a numerical value
• Scalar quantities

  • Speed
  • Distance
  • Mass
  • Temperature
  • Time
• Vector quantities

  Physical quantities that have both a magnitude and a direction
• Vector quantities

  • Velocity
  • Displacement
  • Acceleration
  • Force
  • Momentum
• Representing vectors

  1. Use arrows

  2. Length of arrow indicates magnitude

  3. Direction arrow is pointing indicates direction
• Distance (scalar)

  Doesn't give any idea of direction
• Displacement (vector)

  Gives both magnitude and direction
• Negative vectors can be represented as the opposite direction
• Free body diagrams

  Simple diagrams that show all the forces that are acting on a particular object
• Drawing free body diagrams

  1. Add force arrows to represent all the forces acting on an object

  2. Each force arrow has a magnitude and direction
• Forces are vectors, so they have both magnitude and direction
• Some forces will cancel each other out
• Resultant force

  The overall force on an object after taking into account all the individual forces
• Calculating the resultant force

  1. Look at the horizontal and vertical components separately

  2. Calculate the overall size and direction of the resultant force in each case
• Equilibrium

  When there is no resultant force acting on an object
• Scale drawing

  A drawing where the size of objects is proportional to their actual size
• Finding resultant force on an object using scale drawings

  1. Represent forces as arrows

  2. Place arrows tip to tail

  3. Draw line from start to end

  4. Measure length of line

  5. Convert length to force using scale
• Resultant force

  Magnitude and direction
• Forces balance

  Object is at equilibrium
• Resolving vectors

  1. Draw horizontal and vertical components

  2. Measure lengths of components

  3. Convert lengths to forces using scale
• Applying a force to an object

  Can cause it to compress, stretch or bend
• Deformation

  When an object changes shape
• Elastic deformation

  Object returns to original shape after forces removed
• Inelastic/Plastic deformation

  Object doesn't return to original shape, stays deformed
• Extension

  Increasing length of a spring when stretched
• As force on spring increases

  Extension increases proportionally
• Spring constant (k)


  Measure of how much force required to stretch spring by 1 meter
• Hooke's law

  • Force and extension are directly proportional
  • Deformation is elastic