P5: 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</b>
• Vector quantity


  A quantity that has both magnitude and direction
• Types of forces


  • Contact forces
  • Non-contact forces
• Contact forces


  • The two objects are physically touching
• Normal contact force/Reaction force

  The equal and opposite force exerted by a surface to keep an object in position
• Non-contact forces


  • The two objects do not have to be touching
• Types of non-contact forces

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


  The surrounding area where non-contact forces can act on objects
• The strength of non-contact forces decreases as the objects get further apart
• Scalar quantities


  Physical quantities that only have a magnitude but no direction
• 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 doesn't give any idea of direction, which is why it's a scalar quantity
• Displacement has both a magnitude and a direction, which is why it's a vector quantity
• Free body diagram


  Simple diagram that shows all the forces acting on a particular object
• Drawing a free body diagram

  1. Add force arrows to represent all the forces acting on the 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 the cancellation of 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
• Calculating the resultant force

  • Vertical component: 80,000N up - 80,000N down = 0N
  • Horizontal component: 120,000N right - 90,000N left = 30,000N right
  • Overall resultant force: 30,000N to the right
• If air resistance is 120,000N


  Horizontal resultant force is 0N
• 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 using scale drawing

  1. Represent forces as arrows

  2. Arrange arrows tip to tail

  3. Measure length of resultant vector

  4. Convert length to force using scale
• Forces on object


  • 4 newtons north
  • 3 newtons east
• Arranging forces tip to tail


  Gives resultant force
• Resultant force is 5 newtons at 37 degrees from north
• If forces balance, object is at equilibrium
• Resolving a vector into components

  1. Draw scale diagram of force

  2. Draw horizontal and vertical lines

  3. Measure lengths of horizontal and vertical components

  4. Convert lengths to forces using scale
• Resolving 50 newton force on ramp

  • 40 newtons horizontal
  • 30 newtons vertical
• Elasticity


  The ability of an object to change shape when a force is applied and then return to its original shape when the force is removed
• Elastic deformation


  When an object returns to its original shape after the forces have been removed
• Inelastic/Plastic deformation


  When an object doesn't return to its original shape and stays deformed after the forces have been removed
• Extension


  The increasing length of a spring when it's stretched
• The natural length of a spring is slightly shorter than its full length due to the spring's own weight