Forces

• Vector


  Has magnitude and direction
• Scalar


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


  • Speed
  • Distance
  • Time
  • Energy
  • Mass
• Vectors


  • Velocity
  • Displacement
  • Acceleration
  • Momentum
  • Force
• Displacement above the cliff

  Positive
• Displacement below the cliff

  Negative
• Speed is only velocity when given a direction
• Car travelling round a roundabout at constant speed

  Direction is constantly changing, so velocity is constantly changing, therefore it is accelerating
• Vectors


  • Can be represented by arrows, with their size/length representing the vector magnitude
• Types of forces


  • Non-Contact (Electrostatic, Gravitational attraction)
  • Contact (Normal contact force, Friction)
• All matter has a gravitational field, and attracts all other matter
• Weight


  The force exerted on a mass by the gravitational field, in Newtons
• Weight is measured by a force meter (also known as calibrated spring balance)
• Same person on two different planets

  Their mass is the same, but the gravitational field strength will be different, so their weight will be different
• Acceleration in free fall is due to gravity, and is the same as g (9.8 m/s^2)
• The weight of an object is considered to act at the object's centre of mass
• Resultant Force


  A single force representing the sum of all the forces acting on an object
• Finding the resultant force

  If more than one force act along a straight line, the resultant can be found by adding (acting in the same direction) or subtracting (acting in opposite directions) them
• Initially, the skydiver has no air resistance and the only force acting on him is weight
• Falling in a fluid

  1. Initially, the object will fall freely under gravity 9.8 m/s

  2. Drag forces will act

  3. Object will move at terminal velocity
• As air resistance increases


  The resultant force from weight decreases
• As acceleration decreases


  The object is not speeding up as quickly
• When the resultant force is 0

  There is no acceleration, they travel at terminal velocity
• Free Body Diagrams


  Show the forces (and their directions) acting on an object
• Work


  Work Done = Force x Distance
• Work done is when energy is transferred from the object doing the work to another form
• One joule of work is done when a force of one newton causes a displacement of one metre
• Work done against frictional forces causes a rise in temperature of the object
• Deformation


  Changing the shape of an object by applying forces in opposite directions
• With higher elevation, there are fewer air molecules above the unit area than the same area at lower heights, so there is a smaller weight, so less pressure
• Distance


  How far an object moves, does not involve direction, a scalar quantity
• Displacement


  The distance an object moves in a straight line from start to finish, including direction, a vector quantity
• Speed


  How fast an object is moving, does not involve direction, a scalar quantity
• Velocity


  Speed in a given direction, a vector quantity
• If an object was travelling in a circular motion, the velocity is constantly changing, so there is acceleration due to the changing direction
• The speed of a moving object is rarely constant
• Average speed for non-uniform motion

  Total distance / Total time
• Displacement-Time Graph


  • Gradient is velocity
  • Sharper gradient means faster speed
  • Negative gradient is returning to starting point
  • Horizontal line means stationary
  • Distance means back to starting point
  • Area under line is nothing
  • Curved line means velocity is changing (acceleration)
• Velocity-Time Graph


  • Gradient is acceleration
  • Sharper gradient means greater acceleration
  • Negative gradient is deceleration
  • Horizontal line means constant speed
  • Zero velocity means stationary
  • Area under line is distance travelled
  • Curved line means acceleration is changing