Physics P4 (OCR21)

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  • Created by: Alex
  • Created on: 07-05-10 09:40


  • Speed = Distance/Time
  • EXAMPLE: A cat skulks 20 metres in 40 seconds. Find: a) its speed, b) how long it will take to skulk 75m.
    • a) speed = 20/40 = 0.5ms^-1
    • b) time = 75m/0.5ms^-1 = 150s
  • Pretty rare in real life for an object to stay at exactly the same speed for a long period of time.
  • Usually want to find the average speed if the speed varies constantly for a long period of time.
  • Speed cameras take an instantaneous speed of a car: Evenly spaced lines are printed on the road and the speed camera measures the time in which the car travels over the lines.


  • On a distance time graph:
    • Gradient = speed
    • Flat sections = stationary
    • Steep gradient = faster speed
    • 'Downhill' = travelling in opposite direction
    • Curves = Acceleration/deceleration
    • Steepening curve = speeding up
    • Levelling off curve = slowing down
  • Gradient = Δy/Δx
  • Always use standard units: m, kg, l etc.
  • Distances can be positive or negative.
  • 0 is always start point, +ve = one direction, --ve = other direction.
  • Speed is the gradient of a distance/time graph.
  • Speed = how fast something is going, it does not have a direction .
  • Velociy describes the speed and direction of an object.
  • Speed = scalar quantity (mass, temperature, time, length).
  • Velocity = vector quantity (force, acceleration, momentum).


  • Velocity can be positive or negative.
  • Travel one direction at 20ms^-1, turn around and travel in the opposite direction at -20ms^-1.
  • If two objects heading in opposite directions, one can be said to have positive veolcity while the other have a negative velocity.
  • On a velocity/time graph:
    • Gradient = acceleration
    • Flat sections = steady speed
    • Steeper sections = greater acceleration/deceleration
    • 'Uphill' = acceleration
    • 'Downhill' = deceleration.
    • Area under any section (or all of) graph is equal to the distance travelled in that time interval.
    • Curve = changing accelerations
  • Tachographs plot speed/time when direction isn't important.
  • Tachographs are found in lorries to tell managers how long a driver has gone without a break or if they have been speeding.


  • Forces occur when two objects interact
  • When an object exerts a force on another subject, it interacts with an opposing force: an 'interaction pair'.
    • If you push against a wall, the wall will push back just as hard.
    • As soon as you stop pushingthe wall, so does the wall.
    • If there was no opposing force, you and the wall would fall down.

    • If you exert a force of 10N, the wall's results force will be 10N.
  • NEWTON'S THIRD LAW!: If object A exerts a force on object B, then object B exerts an equal and opposite force on object A.
  • Moving object usually experience friction.
  • When an object is moving relative to another, both objects experience a force in the direction that opposes the movement -- FRICTION!
    • Friction between solid surfaces which are gripping (static).
      • The Earth's tectonic plates trying to move but friction is so strong they stay put.
    • Friction between solid


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