motion and forces

• scalar quantities

  quantity only has magnitude (size), doesn't have direction

  e.g. temperature, time, mass, speed, distance and energy
• vector quantities

  quantity which has both magnitude and direction

  e.g velocity, displacement and force
• velocity - speed of an object in a specific direction
• acceleration ($m/s^2$) = Δv/t (change in velocity / time)

  velocity - (m/s) and time (s)
• $2as =$$v^2 - u^2$

  2 x acceleration x distance = final velocity - initial velocity
• typical value for speed of :

  • sound = 330 m/s
  • walking human = 1.5 m/s
  • running human = 3 m/s
  • cycling human = 6 m/s
  • GPE if not given, assume it's 10 $m/s^2$
• resulatant force = The single force that replaces two or more forces acting on an object
• newtons 3 laws :

  1. (inertia) An object at rest remains at rest, and an object in motion remains in motion at constant speed and in a straight line unless acted on by an unbalanced force.
  2. force = mass x acceleration
  3. for every action there is an equal and opposite reaction
• weight = force which acts on object due to gravity + mass of obejct
• weight (N)= mass x gravitational field strength (N/kg)
• when object changes direction, moves in a circle, the object has a constant speed but not velocity
• inertial mass = measure of how difficult it is to change a given object's velocity
• momentum (kgm/s) = mass x velocity
• total momentum before and after a collision are equal
• change in momentum = force x time
• reaction times of humans :

  • use a ruler drop test
  • person A and B hold each end of a ruler with 0cm mark at bottom
  • person A drops the ruler without telling person B
  • person B catches it
  • distance travelled corresponds to their reaction time
• stopping distance = thinking distance = braking distance

  • typical human reaction time is 0.2 - 0.9 s
• factors affecting drivers reaction time :

  1. tiredness
  2. drugs
  3. alcohol
• factors affecting braking distance :

  1. adverse wet/icy road conditions
  2. poor tyre/brake conditions
• when vehicle undergoes very large deceleration ...

  1. kinetic energy converts to thermal, causing brakes to overheat
  2. loss of control over vehicle
• speed = distance/time
• what is the average wind speed ?

  5 - 7 m/s
• what is the average speed of sound ?

  340 m/s
• what is the average speed of walking ?

  1.5 mph
• what is the average speed of running ?

  3 mph
• what is the average speed of cycling ?

  6 mph
• • momentum is always conserved in a collision
• what is the average mass of a car ?

  1500 kg
• • work done to stop a vehicle is the same as the intial kinetic energy of a vehicle