Topic 2 - Motion and Forces

• vectors

  • has magnitude and direction
  • can be negative
  • velocity, acceleration, displacement, force, weight, momentum
• scalars

  • only magnitude
  • cant be negative
  • speed, distance, energy, mass, temperature, time
• displacement

  • measures the distance and direction from an objects starting point to its finishing point
  • the direction could be relative to the point or a bearing
• speed and velocity

  • speed = how fast your going with no regard to direction - 30mph
  • velocity = speed in a given direction - 30mph north
• acceleration

  • change in velocity in a certain amount of time
  • deceleration - negative acceleration - if something slows down the change in velocity is negative
  • uniform acceleration - constant acceleration
  • acceleration due to gravity is uniform for objects in free fall - roughly 10m/s^2 near earths surface and has the same gravitational field strength
• displacement time graphs

  • gradient = velocity
  • sharper gradient = faster speed
  • horizontal line = stationary
  • 0 distance = back to starting point
  • area under line = nothing
  • curved line = acceleration - velocity changing
• velocity time graphs

  • gradient = acceleration
  • sharper gradient = greater acceleration
  • horizontal line = constant speed
  • 0 velocity = stationary
  • area under line = distance travelled
  • curved line = acceleration is changing
• newtons first law

  an object has a constant velocity unless acted on by a resultant force

  • if resultant force acts on object it will accelerate
  • acceleration = change in velocity over time - velocity will change - direction, speed, or both of object will change
  • if resultant force is 0
  • no acceleration
  • moving at constant velocity - same speed and direction
  • or object is at rest - no speed
• newtons second law

  • force = mass X acceleration
  • F = ma
• weight

  • the force acting on an object due to gravity (the pull of the gravitational force on the object)
  • close to earth this force is caused by the gravitational field around it
  • measured in newtons
  • force is acting from a single point on the object called its center of mass (assume where whole mass is concentrated)
  • measured using calibrated spring balance or newton meter
• circular motion

  • object moving in a circle with a constant speed
  • speed is constant but direction always changing
  • so velocity is always changing
  • so its accelerating
  • this means there must be a resultant force acting on it - force acts towards center of circle
  • centripetal force - force that keeps something moving in a circle
• investigating motion practical

  • measure mass of trolley, unit masses, and hanging hook
  • measure length of card that will interrupt light gate beams
  • adjust height of ramp until trolley starts to move
  • mark line on ramp just before first light gate - makes sure trolley travels same distance each time - light gate will record initial speed as it begins to move
  • attach trolley to hanging mass by string
  • hold still and let go so it starts to roll down slope
  • each light gate will record time trolley passes through it and its speed at that time
  • find acceleration (change in speed / time)
  • initial speed - first light gate - roughly 0m/s
  • final speed - second light gate
  • time - how long it takes to travel between the gates
• to investigate effect on trolleys mass

  • add masses one at a time to trolley
  • keep mass on the hook constant - accelerating force is constant
  • repeat steps
• to investigate effect on accelerating force

  • start with all masses loaded onto trolley
  • transfer masses to hook one at a time
  • repeat steps every time you move a mass
• force and acceleration are

  • proportional
  • as accelerating force increases - acceleration increases
• mass and acceleration are

  • inversely proportional
  • as mass increases - acceleration decreases
• inertia

  • tendency to keep moving with the same velocity
  • inertial mass - measures how difficult it is to change velocity of an object
  • mass = force / acceleration
• newtons third law

  • when two forces interact the forces they exert on each other are equal and opposite
  • weight of book on table = reaction force on book by table
  • rocket taking off - force of gases being ejected = force that lifts rocket from surface
  • two marbles colliding - force exerted by one marble = force exerted by the other
• momentum

  • product of an objects mass and velocity
  • property that all moving objects have
  • greater the mass / velocity - more momentum
  • vector quantity
• total momentum before = total momentum after

  • in closed system - collisions
  • conservation of mass - total momentum before an event is same as after
  • momentum is always conserved in a collision
• ruler drop experiment - human reaction time

  • sit with arms resting on table - not moving up or down
  • someone holds ruler above your open hand
  • they drop it without warning at a random time
  • close thumb and finger to try catch ruler as quick as possible
  • record distance from bottom of ruler to point where it was caught
  • average this - hard to do accurately
  • longer distance = longer reaction time
• thinking distance

  • distance car travels in drivers reaction time - time between noticing hazard and applying brakes
  • affected by:
  • reaction time - increased by tiredness, alcohol, drugs, distractions
  • speed - faster your going teh further youll travel during reaction time
• braking distance

  • distance taken to stop once brakes have been applied
  • affected by:
  • speed - faster your going longer it takes to stop
  • mass of car - more mass means itll take longer to stop
  • condition of brakes - worn or faulty wont brake with as much force
  • amount of friction between tires and road - more likely to skid if road is dirty, icy, wet
• dangers of large decelerations

  • in crash - large deceleration in short period of time as you stop moving from a high speed
  • force = ma - so the large deceleration means a great force exerted on the car and passangers
  • force can cause injury
  • before crash - large momentum - high velocity
  • after - no momentum
  • force = change in momentum over time - so great force is felt
  • average mass of a car - about 1500kg