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 (e.g. tan)
• Balanced forces


  Forces that add up to zero, so 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

  The upward force must equal the weight of the object
• Calculating work done


  Work done = Force * Distance moved
• 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


  Measure of the stiffness of a spring, in N/m
• Force and extension are directly proportional for an elastic object
• Energy stored in a spring

  Equal to 1/2 * Spring constant * (Extension)^2
• Moment


  A turning force, equal to force * perpendicular distance to pivot
• 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


  Due to collisions of gas particles with surfaces
• Increasing gas pressure


  Can be done by adding more gas, reducing volume, or increasing temperature
• Decreasing atmospheric pressure


  As altitude increases, due to fewer gas particles per volume
• Speed and velocity


  Speed is scalar, velocity is vector (has direction)
• Calculating speed and velocity

  Speed = Distance / Time

  Velocity = Displacement / Time
• Acceleration


  Rate of change of velocity, calculated as (Final velocity - Initial velocity) / Time
• Acceleration due to gravity on Earth is 9.8 m/s^2
• Using velocity-time graphs


  Gradient = Acceleration

  Area under graph = Displacement
• Newton's equations of motion

  Relate displacement, initial velocity, final velocity, acceleration, and time
• AQA only provide one of the Newton's equations of motion in the formula sheet
• Newton's first law


  An object's motion is constant if the resultant force is zero
• 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 and 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 * Velocity, a vector quantity
• Momentum is conserved in collisions, even if kinetic energy is not