topic 2 - motion & forces

• scalars

  quantities with only magnitude

  example : mass, speed, distance, energy, time, power
• vectors

  quantities with magnitude and direction

  example : displacement, velocity, momentum, force, acceleration, weight
• velocity

  the rate at which an object changes direction
• formula for velocity

  velocity = displacement / time
• speed

  the rate at which an object covers distances
• formula for speed

  speed = distance / time
• acceleration

  the rate of change of velocity over time, how quickly the velocity changes
• formula for acceleration

  acceleration = change in velocity / time
• momentum

  a measure of the motion in an object. it describes how difficult it is to stop a moving object
• formula for momentum

  momentum = mass x velocity

  ( p ) ( m ) ( v )
• displacement

  the distance moved in a a straight line in a given direction from the start to the ending point
• distance - time graph

  the gradient tells us the speed

  steeper gradient = faster speed

  flatter gradient = slower speed

  flat line = object stopped
• formula for gradient

  gradient = change in y / change in x
• velocity - time graph

  the gradient tells us the acceleration

  steeper gradient = bigger acceleration

  flatter gradient = smaller acceleration

  totally flat = zero acceleration & constant velocity
• formula for acceleration

  acceleration = change in velocity / time taken for change

  ( m / s^2) ( m / s ) ( s )
• formula for change in velocity

  change in velocity = final velocity - initial velocity

  🔼v = v - u
• formula for acceleration related to distance

  final velocity^2 - initial velocity^2 = 2 x acceleration x distance
• free fall on earth

  10m/s
• freefall

  the rate that gravity will increase per second as you fall
• resultant force

  the total force that results from two or more forces acting upon a single object
• newtons first law of motion

  the velocity of an object will only change if a resultant force is acting on the object
• newtons second law of motion

  explains the relationship between force, mass and acceleration
• formula for force

  force = mass x acceleration

  f = m x a

  (newtons) (kg) ( m / s^2)
• newtons third law of motion

  whenever two objects interact, the forces the exert on each other are equal and opposite
• newtons third law checklist

  -there must be two or forces action on two different objects

  -the two forces must be equal in size snd opposite in direction

  -the two forces must be the same type of force
• laws on momentum

  the total momentum of a system remains constant provided no external force acts on it

  total momentum before collision = total momentum after
• thinking distance


  the distance travelled whilst thinking about breaking
• braking distance


  the distance the vehicle travels after brakes are pressed
• stopping distance

  the total distance travelled by the car in response to an emergency
• formula for stopping distance


  total stopping distance = thinking distance + braking distance
• typical human reaction time

  1 / 4
• factors that increase braking distance

  -worn out brakes / tyres

  -poor weather

  -poor road surface

  -faster speed
• factors that increase thinking distance

  -alcohol

  -drugs

  -distractions

  -tiredness

  -faster speed
• formula for work done when stopping a car

  work done = braking force x distance

  w = f x d
• formula for braking force

  braking force = mv^2 / 2d
• how can the hazards of large decelerations be reduced

  -make the deceleration happen over a larger time to decrease the impact force and reduce damage

  -make the momentum happen over a longer time to decrease the impact force and reduce damage
• how are cars designed to keep passengers safe

  -seatbelts

  -airbags

  -crumble zones

  these features increase the time taken for change of speed of the occupants. this reduces their deceleration, and consequently serious injuries to be reduced