P8: forces in balance

• Scalar quantity


  Quantity that has size but no direction
• Vector quantity


  Quantity that has both size and direction
• Scalar quantities


  • Temperature
  • Mass
  • Distance
  • Speed
• Vector quantities


  • Weight
  • Displacement
  • Acceleration
  • Velocity
• Displacement


  The distance moved in a certain direction
• Distance


  The total distance travelled, including any twists and turns
• Displacement is a vector quantity, distance is a scalar quantity
• Force


  A specific type of vector quantity that involves one object putting a push or pull on another object
• Types of forces


  • Contact forces
  • Non-contact forces
• Non-contact forces


  • Magnetism
  • Gravity
  • Electrostatic forces
• Contact forces


  • Air resistance
  • Friction
• Newton's third law


  When an object exerts a force on another object, it experiences an equal and opposite force
• For a tractor to pull a car out of mud, the force of the tractor on the car must be greater than the force of the car on the tractor
• Free body diagram


  A diagram showing all the forces acting on a particular object
• For an object at rest, the forces acting on it are balanced
• For an object accelerating, the driving force is greater than the friction force
• For an object decelerating, the friction force is greater than the driving force
• The vertical and horizontal components of a resultant force do not always add up
• Calculating resultant force when forces are at an angle

  1. Draw to scale diagram

  2. Measure lengths of force vectors

  3. Use protractor to find angle

  4. Draw parallelogram

  5. Measure resultant force
• Center of mass


  The point at which all the mass of an object can be measured from
• Finding center of mass for asymmetric objects

  1. Hang object from random point

  2. Draw plumb line

  3. Rotate object and repeat

  4. Where lines intersect is center of mass
• Center of mass should be low for stable objects like vehicles