forces phy 2

• Vector

  Quantity with both magnitude and direction
• Scalar


  Quantity with only magnitude
• Examples of scalars

  • Speed
  • Distance
  • Time
  • Energy
  • Mass
• Examples of vectors

  • Velocity
  • Displacement
  • Acceleration
  • Force
  • Momentum
• Scalars cannot be negative, but vectors can be, as a certain direction is positive
• A ball thrown off a cliff

  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
• Velocity

  Speed given a direction
• A car travelling round a roundabout at constant speed

  While its speed is constant, its direction is constantly changing – so its 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)
• Gravitational field

  All matter has a gravitational field, and attracts all other matter
• Weight

  The force exerted on a mass by the gravitational field, in Newtons
• 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
• Skydiver example

  1. Initially, the skydiver has no air resistance and the only force acting on him is weight

  2. As he falls, he accelerates, increasing his speed

  3. As air resistance increases, the resultant force from weight decreases

  4. Eventually they are equal and balance, so there is no resultant force

  5. So there is no acceleration when the resultant force is 0 they travel at terminal velocity

  
• Free body diagram

  Shows the forces (and their directions) acting on an object

  
• Work done

  Energy transferred from the object doing the work to another form
• Work done against frictional forces causes a rise in temperature of the object
• Deformation

  Changing the shape of an object
• Types of deformation

  • Elastic (object returns to original shape)
  • Plastic (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
• Equilibrium is when: sum of anticlockwise moments = sum of clockwise moments
• Gears

  Can change speed, force or direction by rotation
• Gear system

  1. If connected to a gear with fewer teeth, the second gear will turn faster but with less force, in opposite direction

  2. If connected to a gear with more teeth, the second gear will turn slower but with more force, in opposite direction
• The second gear will always turn in the opposite direction to the first gear
• Pressure

  Force per unit area
• 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 is the force which causes the pressure in the atmosphere
• Upthrust

  A partially (or totally) submerged object experiences a greater pressure on the bottom surface than on the top surface, creating a resultant force upwards
• 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, does not involve direction, 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, a vector quantity
• Speed

  Does not involve direction, a scalar quantity