forces (p5)

• Vector

  Quantity with both magnitude and direction
• Scalar


  Quantity with only magnitude
• Scalars cannot be negative, but vectors can be, as a certain direction is positive
• Examples of scalars and vectors

  • Speed (scalar)
  • Velocity (vector)
  • Distance (scalar)
  • Displacement (vector)
  • Time (scalar)
  • Acceleration (vector)
  • Force (vector)
  • Mass (scalar)
  • Momentum (vector)
  • Energy (scalar)
• Vectors can be represented by arrows

  The size/length representing the vector magnitude
• Types of forces

  • Non-contact forces (electrostatic, gravitational attraction)
  • Contact forces (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)
• On Earth, g = 9.8
• 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
• 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
• Skydiver example

  1. Initially, only weight acts (acceleration)

  2. As air resistance increases, resultant force from weight decreases (acceleration decreases)

  3. Eventually air resistance and weight balance, so there is no resultant force (terminal velocity)
• Free body diagrams show the forces (and their directions) acting on an object

  
• Resolving forces

  A force F at angle θ to the ground can be resolved parallel and perpendicular to the ground using Pythagoras' Rule
• Work done

  Energy 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
• Types of deformation

  • Elastic deformation (object returns to original shape)
  • Plastic deformation (object does not return to original shape)

  
• Hooke's law

  The extension of an elastic object is directly proportional to the force applied, provided the limit of proportionality is not exceeded
• Force-extension graph

  • Linear line in elastic region following Hooke's law, gradient is k
  • Non-linear line in plastic region not following Hooke's law
• Moment of a force

  Force x perpendicular distance from pivot
• Equilibrium is when the sum of anticlockwise moments equals the sum of clockwise moments
• Gears

  • Can change speed, force or direction by rotation
  • Smaller gear turns second gear faster with less force,
  • larger gear turns second gear slower with more force
  • Second gear always turns in opposite direction to first gear
• Pressure

  Force per unit area
• Pressure produces a net force at right angles to any surface
• Pressure in a fluid

  Varies with depth and density, leading to an upwards buoyancy force
• An object floats if its weight is less than the weight of the water it displaces
• The atmosphere gets less dense with increasing altitude
• The weight of the air causes the atmospheric pressure
• Upthrust

  A partially (or totally) submerged object experiences a greater pressure on the bottom surface than on the top surface. This creates a resultant force upwards which is known as upthrust
• Earth's Atmosphere

  • A thin layer (relative to size of the earth) of air around the Earth
  • The atmosphere gets less dense with increasing altitude
• The atmosphere is a thin layer (relative to the size of the Earth) of air round the Earth. The atmosphere gets less dense with increasing altitude
• Idealised Assumptions, for a simple model of the atmosphere

  • Isothermal, so it is all at the same temperature
  • Transparent to solar radiation
  • Opaque to terrestrial radiation
• Distance

  How far an object moves. Distance does not involve direction. Distance is a scalar quantity
• Displacement

  Includes both the distance an object moves, measured in a straight line from the start point to the finish point and the direction of that straight line. Displacement is a vector quantity
• Speed

  Does not involve direction. Speed is a scalar quantity
• Velocity

  Speed in a given direction. Velocity is a vector quantity