physics 2

• 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
• Representing forces


  With vectors (arrows showing direction and magnitude)
• Resultant force


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


  1. Technically adding the vectors, with forces in opposite directions being negative

  2. Using Pythagoras if forces are at right angles

  3. Using trigonometry (SOH CAH TOA) to find angles
• Balanced forces


  Forces that add up to zero, meaning the object will not accelerate
• Balanced forces mean the object stays at a constant velocity, which could be 0 m/s
• 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


  Requires a force equal to the object's weight
• Calculating work done


  1. Work done = force * distance moved

  2. For lifting an object, work done = mass * gravitational field strength * height
• Hooke's law


  Force = spring constant * extension
• Spring constant


  The stiffness of a spring, measured in N/m
• The energy stored in a spring is equal to 1/2 * k * (extension)^2
• Moment


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


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


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


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


  Caused by collisions of gas particles with surfaces, increased by adding more gas, reducing volume, or increasing temperature
• Pressure decreases with increasing altitude due to lower atmospheric density
• Velocity


  Speed in a particular direction, can be positive or negative
• Calculating speed and velocity

  Speed = distance / time

  Velocity = displacement / time
• Acceleration


  The rate of change of velocity, measured in m/s^2
• Acceleration due to gravity

  9.8 m/s^2 downwards
• Using equations of motion


  s = ut + 1/2 at^2

  v = u + at

  v^2 = u^2 + 2as
• 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 force
• Newton's second law


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

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

  Measure acceleration with light gates

  Plot force vs acceleration graph, should be straight line through 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 hazard
• Braking distance


  The distance travelled while braking
• Doubling speed


  Quadruples braking distance
• Momentum


  Mass * velocity, a vector quantity
• Momentum is conserved in collisions, even if kinetic energy is not
• Doubling your speed


  Quadruples your braking distance