5.6 Forces and Motion

• Distance quantity

  No specific direction is required so it is a scalar quantity
• The object has zero displacement
• Displacement


  A vector quantity that involves direction
• The object starts and ends at the same point
• Typical value for the speed of sound
• Typical value for human walking speed
• Typical value for human running speed
• Typical value for human cycling speed
• Equation linking distance, speed and time

  Distance = Speed x Time
• Units for distance, speed, and time

  • Distance (m)
  • Speed (m/s)
  • Time (s)
• 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
• Speed from a distance-time graph


  The speed is equal to the gradient of the graph
• To calculate speed at a given time from a distance-time graph for an accelerating object, draw a tangent to the curve at the required time and calculate the gradient of the tangent
• Equation for average acceleration

  Acceleration = (Change in Velocity)/(Time Taken)
• Units for acceleration, velocity, and time

  • Acceleration (m/s²)
  • Velocity (m/s)
  • Time (s)
• Distance travelled from a velocity-time graph

  It is equal to the area under the graph
• Approximate value for the acceleration of an object in free fall under gravity near the Earth’s surface
• The resultant force is zero when an object is falling at terminal velocity
• When at terminal velocity, the object is moving at a constant speed and so isn’t accelerating
• Newton’s first law for a stationary object


  If the resultant force on a stationary object is zero, the object will remain at rest
• Newton’s first law for a moving object

  If the resultant force on a moving object is zero, the object will remain at constant velocity (same speed in same direction)
• The braking forces are equal to the driving forces when a car is travelling at constant velocity
• If an object changes direction but remains at a constant speed, there is a resultant force
• Newton’s first law

  If the resultant force on a moving object is zero, the object will remain at constant velocity (same speed in same direction)
• When a car is travelling at constant velocity

  The braking forces are equal to the driving forces
• If an object changes direction but remains at a constant speed


  There is a resultant force
• Inertia


  The tendency of an object to continue in its state of rest or uniform motion
• Resultant force = Mass x Acceleration
• Newton’s Second Law in words

  An object’s acceleration is directly proportional to the resultant force acting on it and inversely proportional to its mass
• 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
• Stopping distance of a vehicle

  The sum of thinking distance and braking distance
• For a given braking distance, if the vehicle’s speed is increased

  The stopping distance is increased
• Typical range of values for human reaction time

  • 0.2 seconds
  • 0.9 seconds
• Factors affecting a driver’s reaction time

  • Tiredness
  • Drugs
  • Alcohol
• Factors affecting 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