1. forces and motion

• Speed


  Distance travelled per unit time
• Acceleration


  Change in velocity per unit time
• Velocity


  Speed in a given direction
• Units


  • Distance in metres (m), Time in seconds (s), Speed and Velocity in metres per second (m/s), Acceleration in metres per second squared (m/s^2)
• Distance-time graph


  • Gradient is velocity
  • Negative gradient is returning to starting point
  • Horizontal line is stationary
  • Distance zero is back at starting point
  • Curved line is accelerating
• Velocity-time graph


  • Gradient is acceleration
  • Negative gradient is deceleration
  • Speed zero is at rest
  • Horizontal line is constant speed
  • Area under line is distance travelled
  • Curved line is changing acceleration
• Vector


  Has magnitude and direction
• Scalar


  Has just a magnitude
• Examples of scalars and vectors

  • Scalars: Distance, Speed, Time, Energy
  • Vectors: Displacement, Velocity, Acceleration, Force
• Forces


  • Can change speed, shape or direction of a body
  • Measured in Newtons (N)
  • Different types include gravitational, electrostatic
• Friction


  Force between two surfaces that impedes motion and results in heating
• Air resistance


  A form of friction
• Finding resultant of forces


  Add if in same direction, subtract if in opposite direction
• Newton's first law


  • An object has constant velocity unless acted on by a resultant force
• Newton's third law


  • Every action force has an equal and opposite reaction force
• Mass


  Measure of how much matter is in an object, measured in kilograms (kg)
• Weight


  Gravitational force on an object, measured in Newtons (N)
• Gravitational field strength on Earth is 10N/kg
• Weight of an object acts through its centre of gravity
• Motion of a body falling in a uniform gravitational field


  1. Initially no air resistance, only weight force

  2. As it falls, speed increases so air resistance increases

  3. Resultant force downwards decreases

  4. Acceleration decreases

  5. Eventually air resistance and weight balance, so no resultant force and terminal velocity reached
• Thinking distance


  Distance travelled between driver realising need to brake and pressing brakes
• Factors increasing thinking distance

  • Greater speed
  • Slower reaction time due to alcohol, tiredness or distractions
• Braking distance


  Distance travelled between pressing brakes and vehicle stopping
• Factors increasing braking distance

  • Greater speed or mass
  • Poor road or car conditions
• Stopping distance


  Sum of thinking distance and braking distance
• Deformation


  Change in size and shape of a body due to a force
• Elastic deformation


  Object returns to original shape when load removed
• Force-extension graph


  • Linear region follows Hooke's law
  • Limit of proportionality where it stops being linear
  • Non-linear region does not follow Hooke's law, then fractures
• Moment of a force


  Measure of its turning effect, in Newton metres (Nm)
• Equilibrium


  • Sum of clockwise moments equals sum of anticlockwise moments, no resultant force
• For a horizontal beam, upwards forces at supports change with position of heavy object
• Safety features in cars work by increasing time taken to come to rest, reducing force
• Conservation of momentum


  • Total momentum before equals total momentum after a collision