Physics Edexcel GCSE

• Vector


  Has magnitude and direction
• Scalar


  Has just magnitude
• Generally, scalar cannot be negative, but vectors can be, as a certain direction is positive
• Speed


  • Scalar
• Velocity


  • Vector
• Distance


  • Scalar
• Displacement


  • Vector
• Time


  • Scalar
• Acceleration


  • Vector
• Force


  • Vector
• Mass


  • Scalar
• Momentum


  • Vector
• Energy


  • Scalar
• Displacement is 0 at height of cliff, above the cliff the ball has positive displacement, and below the clifftop the ball has negative displacement
• In long answer questions, you may be able to decide where the "0" point of a vector may lie
• Speed


  Only velocity when given a direction
• Displacement Time Graphs


  • Gradient is velocity
  • Sharper gradient means faster speed
  • Negative gradient is returning back to starting point
  • Horizontal line means stationary
  • 0 Distance means that it is back to starting point
  • Area under line = nothing
  • Curved Line means the velocity is changing (acceleration)

  
• Velocity Time Graphs


  • Gradient is acceleration
  • Sharper gradient means greater acceleration
  • Negative gradient is deceleration
  • Horizontal line, constant speed
  • 0 velocity means that it is stationary
  • Area under line = distance travelled
  • Curved Line means that the acceleration is changing

  
• Average Speed


  For when the speed changes during the motion, use overall distances and timings to work out average speed

  
• Methods to Determine Speeds

  1. For constant speeds: Measure distance travelled, use stopwatch for time taken, use speed = distance/time

  2. For average speed: Work out total distance travelled, find the time taken for the whole journey, use speed = distance/time

  3. Using light gates: Set up two, one at start and one at end, measure distance between them, as soon as the object passes through the first, it will measure the time taken to reach the second, then use speed = distance/time

  
• Typical speeds


  • Wind: 5 - 7 m/s
  • Sound: 340 m/s
  • Walking: 5 km/h = ~ 1.4 m/s
  • Running: ~6 miles per hour = ~3 m/s
  • Cycling: 15 km/h = ~4 m/s
  • Bus: 14 km/h
  • Train: 125 miles/h
  • Plane: 900 km/h
• Acceleration due to gravity: g = 10 m/s^2
• Newton's First Law


  • An object has a constant velocity unless acted on by a resultant force
  • If a resultant force acts on the object, it will accelerate
  • If the resultant force is zero, no acceleration, so moving at constant velocity or the object is at rest
• Newton's Second Law


  Force = mass x acceleration
• Weight


  • Measured using a force meter, or weighing scales, and is used to work out mass of unknown object
  • The greater the gravitational field strength, the greater the weight
• Circular Motion


  • Object moving in a circle, with constant speed
  • The speed is constant, but direction always changing
  • So the velocity is always changing
  • So it is accelerating
• Centripetal Force


  For motion in a circle, there must be a force which supplies this acceleration, directed towards the centre of the circle
• Inertial Mass


  A measure of how difficult it is to change the velocity of an object (including from rest), measured by inertial mass = force/acceleration
• Newton's Third Law


  • Every action force has an equal and opposite reaction force
  • A book on a table: The weight of the book on the table = The reaction force on the book by the table
  • Rocket taking off: The force of the gases being ejected from the rocket is equal to the force that lifts the rocket from the surface
  • Collisions: The force exerted by one marble on the other is the same as the force from the other
• Momentum


  Momentum is always conserved in a collision, momentum = mass x velocity
• In collisions, total momentum before = total momentum after
• Human Reaction Time


  • There is a delay between a human observing an event, and acting
  • Average human reaction is 0.25 seconds (250 milliseconds)
• Vehicle Stopping Distances


  • Thinking Distance: Speed, affected by reaction time, concentration, tiredness, distractions, influence of drugs/alcohol
  • Braking Distance: Speed, poor road conditions, bald tires, worn brake pads, mass

  
• Greater the speed, the greater distance travelled during the same time (reaction time)

  
• Dangers of Large Decelerations

  When in a crash, there is a large deceleration over a very short time as you stop moving from a high speed, causing a great force to be exerted on the car and passengers, which can cause injury
• Work Done to Stop


  The work done to stop a vehicle is equal to the initial KE of the vehicle, as all the kinetic energy the car had has to be transferred to friction for it to stop
• Energy Transfer


  • Diagrams show energy input, and the energy output, including the forms that the energy takes and the waste output energy
• Energy Changes


  • Object projected upwards: KE transferred to GPE, then vice versa as it falls back down
  • Object projected up a slope: KE transferred to GPE (and also to heat if friction is present)
  • Moving object hitting an obstacle: KE transferred to sound / KE transferred to obstacle if that moves too
  • Object being accelerated by a constant force: Object is having work done to it, with it gaining KE, whatever supplies the force is having its energy transferred to KE
  • Vehicle slowing down: KE transferred to heat (through brakes)
  • Boiling water in kettle: Electrical energy to thermal
• Conservation of Energy


  In a closed system, the total energy in the system never changes, regardless of the energy transfers that take place
• Mechanical Waste Energy


  In mechanical processes, energy transferred to it can cause a rise in temperature, so energy is dissipated to surroundings (heat is transferred to air), making the process wasteful