SP1/2 motion and forces

• scalar 


  only size not direction

  eg. speed, distance , mass and energy
• vector

  size and direction

  eg. velocity, displacement, force, acceleration, weight, and momentum
• objects moving in a circle

  can move at a constant speed, but will always have a changing velocity because the direction they are travelling in is always changing.
• Typical speeds

  walking= 1.5m/s

  running= 3m/s

  cycling= 6m/s

  sound in air= 340m/s

  wind= 5-20m/s

  car= 13-30m/s

  train= 55m/s

  plane= 250m/s

  ferry= 15m/s
• speed equation

  speed= distance ÷ time
• what is the acceleration due to free fall?

  10m/s
• what does the gradient of a distance/time graph show?

  speed
• what does the gradient of a velocity/time graph show?

  acceleration
• what is the area under a velocity/time graph for a given time interval?

  the distance travelled by the object during that time period
• Newton's 1st law


  a resultant force is needed to change the motion (the speed or direction) of an object
• what does a resultant force of zero on a stationary object mean?

  object is still
• what does a resultant force of zero on a moving object mean?

  object's speed or direction of travel will stay the same
• what happens if a non-zero resultant force acts on an object?

  it will start to move, speed up, slow down, stop moving or change direction
• Newton's 2nd law

  the acceleration of an object is directly proportional to the resultant force acting on it and inversely proportional to the object's mass
• objects travelling in a circle means what?

  always accelerating, so a resultant force must always be acting on them. This resultant force always acts towards the centre of the circle- centripetal force
• inertial mass

  describes how difficult it is to change the object's velocity
• weight


  force acting on an object due to gravity

  -directly proportional to the mass of an object and to the gravitational field strength of the gravitational field that the object is in.

  -weight can be measured using a newton meter (spring balance)

  • W= m X g
• Investigating motion


  -investigating newton's second law by changing the mass of and force acting on a trolley

  -light gates: best for measuring short time intervals as it is not affected by reaction times unlike stop watches

  • 30cm ruler: short lengths
  • metre ruler: for larger distances
  • video camera: determine speed by knowing how many frames per second the camera records , you can find the distance travelled by an object in a given number of frames and the time that it takes to do so
• Newton's 3rd law

  when two objects interact, they exert equal but opposite forces on each other. These forces are the same type
• momentum


  property of a moving object that is the product of its mass and velocity

  -it is always conserved in collisions when there is no external force acting on it

  -force is needed to change the momentum
• newton's third law and momentum

  whenever two objects interact they exert equal but opposite forces on each other. Because a force causes a change of momentum, these equal but opposite forces are what lead to momentum being conserved in collisions (when no external force is acting on it)
• stopping distance


  distance covered between the driver spotting a hazard and the vehicle coming to a stop. It is the sum of the braking distance and thinking distance
• thinking distance


  distance the vehicle travels during the driver's reaction time

  -affected by the speed of the vehicle and the driver's reaction time
• braking distance


  distance travelled whilst the vehicle's brakes are applied

  -affected by the speed and mass of the vehicle, how good the brakes are, how good the grip is between the vehicle and the road, condition of the tyres and the state of the road
• driver's reaction time affected by what?

  speed of the vehicle and the driver's reaction time
• what happens when a vehicle brakes?

  friction does work, and transfers energy away from the kinetic energy store of the vehicle.
• what happens when a car is brought to a stop?

  the work done during braking is equal to the energy in the kinetic energy store of the car just before the brakes were applied
• why are large decelerations dangerous?

  because they exert large forces on passengers which can cause injuries, and they can cause brakes to overheat, and vehicles to skid.
• typical human reaction time

  0.2-0.9 seconds