Motion and Safety

• Constant speed


  When an object is travelling at a constant speed but not a constant velocity
• When an object is travelling at a constant speed but not a constant velocity
• Circular motion


  • Speed is a scalar quantity
  • Velocity is a vector quantity which means it can only be constant if the direction is constant
  • In circular motion, the direction is continuously changing
  • Velocity is constantly changing
• Centripetal force


  The resultant force that acts on an object moving in a circle, and it acts towards the centre of the circle
• Inertial mass


  • A measure of how difficult it is to change a given object's velocity
  • The ratio of force over acceleration
• Newton's Third Law


  Whenever two objects interact, the forces that they exert on each other are always equal and opposite
• Momentum


  Mass x Velocity
• The unit used for momentum is kgm/s (kilogram metres per seconds)
• In a closed system, the total momentum before and after a collision are equal
• Equation linking change in momentum, force and time


  • Force x Time = Change in Momentum
  • F Δt = mΔv
• Measuring human reaction times


  1. Using the ruler drop test

  2. Person A and B hold each end of a ruler with the 0 cm mark at the bottom

  3. Person A drops the ruler without telling person B

  4. Person B catches it

  5. The distance travelled corresponds to their reaction time
• Importance of 0cm mark at the bottom


  So you can obtain the distance directly without having to calculate it; otherwise, a zero error would need to be accounted for
• Stopping distance


  The sum of thinking distance and braking distance
• If the vehicle's speed is increased

  The stopping distance is increased
• A typical range of values for human reaction time is 0.2 seconds - 0.9 seconds
• Factors which can affect a driver's reaction time


  • Tiredness
  • Drugs
  • Alcohol
• Factors which may affect braking distance

  • Adverse (wet/icy) road conditions
  • Poor tyre/brake conditions
• Energy transfers when a car applies its brakes

  1. Work is done by the friction force between the brakes and wheel

  2. Kinetic energy of the wheel is converted to heat and is dissipated to the surroundings through the brake discs
• To stop a car in a given distance, if its velocity is increased

  The braking force must also be increased
• Consequences of a vehicle undergoing very large decelerations


  • Kinetic energy converted to heat is very high causing brakes to overheat
  • Loss of control of the vehicle