Phy P2 vid

• Force


  Any push or pull
• Types of forces


  • Contact forces (when objects are physically touching)
  • Non-contact forces (like magnetism, electrostatic forces, gravity)
• Contact forces


  • Normal contact force (pushing a door)
  • Friction
  • Air resistance
  • Tension
• Finding resultant force


  1. Technically adding vectors

  2. If in opposite directions, one is negative

  3. If at right angles, use Pythagoras or trigonometry
• Balanced forces


  Forces add up to zero, object will not accelerate but may be moving at constant velocity
• Scalar


  Quantity with magnitude but no direction
• Vector


  Quantity with both magnitude and direction
• Scalar quantities examples 


  • Displacement (m)
  • Velocity (m/s)
• Weight


  Force due to gravity acting on an object, calculated as mass x gravitational field strength
• Lifting an object at constant speed

  Lifting force must equal weight
• Work done


  Energy transferred by a force, calculated as force x distance moved
• Gravitational potential energy


  Energy gained when an object is lifted, calculated as mass x gravitational field strength x height
• Hooke's law


  Force = spring constant x extension, for elastic objects
• Energy stored in a spring


  Equal to 1/2 x spring constant x (extension)^2
• Moment


  Turning force, equal to force x perpendicular distance to pivot
• Pressure


  Force per unit area, calculated as force/area
• Pressure in liquids


  Pressure = depth x density x gravitational field strength
• Gas pressure


  Due to collisions of gas particles with surfaces, increased by more particles, smaller volume, or higher temperature
• Velocity


  Speed with direction, measured in m/s
• Acceleration


  Change in velocity over time, measured in m/s^2
• Newton's first law


  An object's motion is constant if there is no resultant force
• Newton's second law


  Force = mass x acceleration
• Proving Newton's second law

  Use a trolley on a track, measure acceleration with light gates, change weight on string, plot force vs acceleration graph
• Newton's third law


  For every action force, there is an equal and opposite reaction force
• Doubling speed


  Quadruples braking distance
• Momentum


  Mass x velocity, a vector quantity
• In a collision, total momentum is always conserved
• Doubling your speed


  Quadruples your braking distance
• Your car needs to lose all of its kinetic energy which is equal to half MV squared
• If you double the velocity (V)

  Kinetic energy goes up by 2^2 = 4 times
• If you triple your speed

  Kinetic energy goes up by 3^2 = 9 times
• Factors that affect thinking distance


  • Distractions
  • Alcohol
  • Drugs
• Factors that affect braking distance


  • Condition of brakes
  • Tires
  • Road
  • Weather
• Momentum


  Measure of how hard it is to get something to stop
• Momentum


  Mass times velocity
• Momentum is a vector, so it can be negative if velocity is negative
• In a collision, kinetic energy isn't always conserved but total momentum always is
• Calculating momentum before and after a collision


  1. M1 U1

  2. M2 U2

  3. M1 V1 + M2 V2
• If nothing is moving before, total momentum before is zero
• If objects are coupled together, total momentum is M * V