P1 - Forces

• Stopping distance


  Thinking distance + braking distance
• Resultant force (Newton's 2nd law)


  force = mass x acceleration

  f=ma
• Common speeds

  Walking = 1.4 m/s

  Running = 3 m/s

  Cycling = 5.5 m/s
• Common speeds

  Cars = 13-31 m/s

  Aeroplanes = 250 m/s

  Trains = 55 m/s
• Common speeds

  Sounds in air = 330 m/s

  Light = 3 x 10^8
• Speed equation


  speed = distance/time
• Acceleration equation

  acceleration = final velocity - initial velocity / time
• Uniform acceleration

  v^2 - u^2 =2ax

  final velocity^2 - initial velocity^2 = 2 x acceleration x distance
• Momentum equation

  p=mv

  momentum (kg m/s) = mass x velocity
• Weight

  w=mg

  weight (N) = mass x gravity (N/kg)
• Scalar quantity


  Magnitude but no direction
• Vector quantity 


  magnitude and direction
• Vector quantities 


  Displacement, velocity, acceleration, force, weight and momentum
• Scalar quantities


  Distance, speed, mass and energy
• Velocity 


  Speed in a stated direction
• Speed(m/s) = distance (m)/time (s)
• On a distance/time graph determine speed by the gradient
• Acceleration (m/s^2) = change in velocity (m/s) / time (s)
• v^2(m/s^2) - u^2(m/s^2) = 2 x a(m/s^2) x X(m)
• Velocity time graphs

  • Calculate the acceleration from the gradient and distance travelled by the area under the graph
• Measure speed in the lab 


  • Measure distance and time
  • Light gates are used as they are more accurate
• The acceleration of free fall/due to gravity is 10m/s^2
• Newton's First Law

  Resultant force on a body is zero

  • Moving at a constant velocity or is at rest

  Resultant force is not zero

  • eg the speed and/or direction changes
• Newton's Second Law 


  F=ma

  Force = N

  Mass = kg

  Acceleration= m/s^2
• Weight

  Force due to the pull of gravity on an object
• Weight(N) = Mass(kg) x Gravitational Field Strength(N/kg)
• Weight is measured in Newtons
• The greater the gravitational field strength the greater the mass and weight of the object
• An object moving in a circular orbit at constant speed has a changing velocity because it is constantly changing direction
• Motion in a circle 


  There must be a resultant force known as centripetal force that acts towards the centre of the circle
• Inertial mass 


  • Measure of how difficult it is to change the velocity of an object
  • Defined as the ratio of force over acceleration
• Newton's Third Law 


  Whenever two objects interact, the exert equal and opposite forces on each other
• An object in equilibrium will not turn or accelerate
• Momentum (kg m/s) = mass (kg) x velocity (m/s)
• Conservation of Momentum

  • Moving objects collide
  • Total momentum of both objects is the same before the collision as it is after the collision
  • As long as there is no external forces acting on it
• F = mv-mu/t

  Force(N) = change in momentum(kgm/s) / time (s)
• Human reaction times

  Measured by asking people to respond as quickly as possible to a stimulus
• Stopping distance = sum of the thinking distance and the braking distance
• Stopping distance of a vehicle is affected by

  • Mass and speed of the vehicle
  • Drivers reaction time
  • State of the brakes
  • State of the road
  • Traction (friction) between the tyre and the road
• Factors affecting a drivers reaction time

  • Drugs,
  • Alcohol
  • Distractions
  • Tiredness
  • Illness