5.6 - Forces and motion

• Distance is how far an object moves.
• Displacement includes both the distance an object moves, measured in a straight line from the start point to the finish point and the direction of that straight line.
• The typical values of speed can include:

  • Walking ~ 1.5 m/s
  • Running ~ 3 m/s
  • Cycling ~ 6 m/s
• Moving objects will always have a constantly changing speed. This also includes the speed of sound and wind. A typical value for the speed of sound in air is 330 m/s
• The velocity of an object is its speed in a given direction.
• Acceleration is the rate at which an object changes its velocity.
• Newton's 1st Law:

  An object continues in its state of rest or constant speed in a straight line unless acted on by a resultant force. This means that when a vehicle travels at a steady speed the resistive forces balance the driving force.
• Inertia is the tendency of an object to continue moving with a constant velocity until it experiences a net external force.
• Newton's 2nd law:

  The acceleration of an object is proportional to the resultant force acting on the object, and inversely proportional to the mass of the object.
• Newton's 3rd law:

  Two interacting objects exert equal and opposite forces on each other e.g. A book pushing on a table as the table pushes on the book. Keeping both objects in equilibrium
• The thinking distance is the distance the vehicle travels during the driver's reaction time
• The braking distance is the distance the vehicle travels whilst the driver is braking
• The stopping distance is the thinking distance + the braking distance
• A vehicle's brakes transforms the kinetic energy of the vehicle to thermal energy using friction. Brakes will get very hot so need to dissipate this thermal energy efficiently to the surroundings. A large deceleration may lead to brakes overheating and/or loss of control
• The braking distance of a vehicle can be affected by:

  • Adverse road
  • Weather conditions
  • Poor condition of the vehicle