P10 - Force and motion

• The greater the resultant force on an object, the greater the acceleraton
• The greater the mass of an object, the smaller the acceleration for a given force
• force = mass x acceleration
• Intertia


  The tendency of an object in motion to stay in motion
• Terminal velocity


  • For an object falling through a fluid:
  • there are two forces acting - the weight due to gravity and the drag
  • the drag force increases as the object speeds up
  • the resultant force decreases as the drag increases
  • the acceleration decreases as it falls
  • if it falls for long enough, the object will reach a final steady speed
• Terminal velocity


  The constant velocity a falling object reaches when the frictional force acting on it is equal to its weight
• Stopping distance


  the distance a vehicle travels to safely come to a stop
• stopping distance = thinking distance + braking distance
• Thinking distance


  the distance the vehicle travels during the driver's reaction time
• Braking distance


  the distance the vehicle travels once the brakes have been applied
• Speed has a bigger effect on braking distance than thinking distance
• • thinking distance is proportional to the speed of the vehicle
  • braking distance is proportional to speed squared
• Factors affecting braking distance


  • the speed of the vehicle
  • road conditions
  • the condition of the brakes and tires
• Any condition that causes less friction between the tyres and the road increases the braking distance
• Braking


  • When the brakes of a vehicle are applied, a frictional force is applied to its wheel
  • Work done by the frictional force between the brakes and the wheels transfers energy from the kinetic energy stores of the car to the thermal stores of the brakes
  • This increases the temperature of the brakes
• energy transfer (J) = braking force (N) x distance (m)
• The faster the vehicle moves or the greater its mass


  • the greater the energy in its kinetic energy store
  • the more work that has to be done to transfer the energy to slow it down
  • the greater the braking force needed to stop it in a certain distance
  • the greater the distance needed to stop it with a certain braking force
• Factors that affect reaction time


  • tiredness
  • drugs
  • alcohol
  • distractions
• Ruler drop test


  Drop a ruler between someone's fingers and the distance it fells before they catch it before they catch it is used to calculate their reaction time
• Momentum


  • if an object is moving or able to move, an unbalanced force acting on it will change its momentum
• force = (change in momentum x velocity) / time
• The greater the time taken for the change in momentum:

  • the smaller the rate of change of momentum
  • the smaller the force it experiences
• The force acting on an object is equal to the rate of change of momentum of the bject
• Vehicle safety features


  Increase the time taken for the change in momentum
• Vehicle safety features


  • air bags
  • seat belts
  • crumple ones
  • cycling helmets
  • crash mats
• Deceleration


  • the greater the braking force, the larger the deceleration