P10 - Forces and motion

• Acceleration of an object

  • Depends on the size of the resultant force acting upon it

  
• Effect of mass on acceleration

  • The greater the mass of an object, the less it accelerates for a given force

  
• Calculating resultant force


  From acceleration and mass

  
• Inertia


  The tendency of an object to stay at rest or to continue in uniform motion
• Rearranging the equation F=ma gives a=F/m or m=F/a
• Newton's second law: the acceleration of an object is proportional to the resultant force on the object and inversely proportional to the mass of the object

  
• The greater the resultant force on an object, the greater the object's acceleration
• The greater the mass of an object, the less it accelerates for a given force
• The resultant force acting on an object is F=ma
• If an object is released above the ground, it falls due to its weight (the force of gravity)
• An object falling freely accelerates downwards at a constant acceleration of 10m/s^2 (the acceleration due to gravity, g)
• Weight


  The force acting on an object due to gravity
• Mass


  The quantity of matter in an object
• Weight = mass x gravitational field strength
• The Earth's gravitational field strength at the surface is about 9.8N/kg
• Terminal velocity


  The constant velocity reached by an object falling in a fluid when the frictional force equals the weight

  
• When an object reaches terminal velocity, the resultant force on it is zero and its acceleration is zero

  
• The gravitational field strength on the Moon is 1.6N/kg
• A parachutist reaches a constant speed when the air resistance force equals the weight
• When an object falls at terminal velocity, the air resistance force equals the weight
• The ratio of the drag force to the weight at 0.15s is about 0.5
• For a vehicle travelling at constant velocity, the resultant force is zero as the driving force equals the resistive forces
• The braking force needed to stop a vehicle depends on its speed and mass
• Reaction time can be tested by catching a falling ruler, typical times range from 0.4s to 0.9s
• SO mph


  50 mph
• jaa


  (13 m/s)
• fj


  (22 mis)
• SOROS


  Forces on the road
• For any car travelling at constant velocity, the resultant force on it is zero
• This is because the driving force of its engine is balanced by the resistive forces, which are mostly caused by air resistance
• Accelerator pedal


  Used by car driver to vary the driving force of the engine
• Braking force needed to stop a vehicle in a given distance


  • Depends on the speed of the vehicle when the brakes are first applied
  • Depends on the mass of the vehicle
• The greater the speed


  The greater the deceleration needed to stop the vehicle within a given distance
• The greater the mass


  The greater the braking force needed for a given deceleration
• Stopping distance


  The shortest distance a vehicle can safely stop in

  
• Thinking distance


  The distance travelled by the vehicle in the time it takes the driver to react (i.e., during the driver's reaction time)

  
• Braking distance


  The distance travelled by the vehicle during the time the braking force acts

  
• Stopping distance = thinking distance + braking distance

  
• Tiredness, alcohol, and drugs affect the brain and increase reaction times
• Distractions like using a mobile phone also increase reaction time and cause serious accidents