Physics paper 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
• Finding resultant force


  1. Technically adding vectors (if in opposite directions, one is negative)

  2. Using Pythagoras if at right angles

  3. Using trigonometry (e.g. tan)
• Balanced forces


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


  Quantity with magnitude but no direction
• Vector


  Quantity with both magnitude and direction
• Scalar quantities


  • Displacement
  • Velocity
• Weight


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

  Upward force must equal weight
• Work done


  Energy transferred by a force, calculated as force x distance moved
• 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
• Gas pressure


  Due to collisions of gas particles with surfaces, increased by adding more gas, reducing volume, or raising temperature
• Speed


  Scalar quantity, distance/time
• Velocity


  Vector quantity, displacement/time
• Acceleration


  Change in velocity/time, 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
• Newton's Third Law


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


  Mass x velocity, conserved in collisions
• Thinking distance

  Distance travelled before reacting to a hazard, proportional to speed
• Braking distance

  Distance to stop after braking, proportional to speed^2
• 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 affecting thinking distance

  • Distractions
  • Alcohol
  • Drugs
• Factors affecting 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 there is zero total momentum before a collision, there must be zero total momentum afterwards
• Recoil is an example of zero total momentum before and after an event
• Force

  Rate of change of momentum
• Shorter time for momentum change

  Bigger force needed