Physics

• 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
• Resultant force


  The net force acting on an object when multiple forces are present
• Finding resultant force


  1. Technically adding the vectors

  2. If forces are in opposite directions, one must be negative
• Finding resultant force (vectors at right angles)

  1. Use Pythagoras

  2. Use trigonometry (SOH CAH TOA)
• Balanced forces


  Forces that add up to zero, so the object will not accelerate
• Scalar


  A quantity with magnitude but no direction
• Vector


  A quantity with both magnitude and direction
• Weight


  The force due to gravity acting on an object, calculated as mass * gravitational field strength
• Gravitational field strength on Earth is 9.8 N/kg (often rounded to 10 N/kg)
• Lifting an object at constant speed

  The upward force must equal the weight of the object
• Gravitational potential energy


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


  Force = Spring constant * Extension
• Spring constant


  The stiffness of a spring, measured in N/m
• Hooke's law applies to any object that stretches or compresses elastically
• Experiment to demonstrate Hooke's law

  1. Hang masses from a spring and measure the extension

  2. Plot a graph of force vs extension, which should be a straight line through the origin
• Moment


  A turning force, equal to force * perpendicular distance to the pivot
• The unit for moment is newton-metres (N·m)
• Principle of moments


  If the clockwise and anticlockwise moments are balanced, the object will not turn
• Pressure


  Force per unit area, calculated as force / area
• Unit of pressure


  Pascals (Pa) or Newtons per square metre (N/m^2)
• Gas pressure


  Caused by collisions of gas particles with surfaces, can be increased by adding more gas, reducing volume, or increasing temperature
• Atmospheric pressure decreases with increasing altitude due to fewer gas particles
• Velocity


  Speed with direction, measured in m/s
• Acceleration


  Rate of change of velocity, measured in m/s^2
• Finding displacement from a velocity-time graph

  The area under the graph gives the displacement
• Newton's equations of motion

  Equations relating displacement, initial velocity, final velocity, acceleration, and time
• Newton's first law


  An object's motion is constant (including 0 m/s) unless acted on by a resultant force
• Inertia


  The tendency for an object's motion to stay constant unless acted on by a resultant force
• Newton's second law


  Force = Mass * Acceleration
• Proving Newton's second law experimentally

  1. Use a trolley on a track, pulled by weights over a pulley

  2. Measure acceleration and plot force vs acceleration graph, which should be a straight line through the origin
• Newton's third law


  For every action force, there is an equal and opposite reaction force
• Thinking distance


  The distance travelled before reacting to a stimulus
• Braking distance


  The distance travelled while braking
• Doubling speed


  Quadruples braking distance
• Momentum


  Mass * Velocity, a vector quantity
• In a collision, total momentum is always conserved, even if kinetic energy is not
• Doubling your speed


  Quadruples your braking distance
• Your car needs to lose all of its kinetic energy which is equal to half MV squared