(P5) FORCES

• 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
• Displacement is 0 at the height of a cliff, above the cliff the ball has positive displacement, and below the clifftop the ball has negative displacement
• Speed


  Velocity when given a direction
• A car travelling round a roundabout at constant speed has a constantly changing velocity, 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
• 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
• 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

  
• Resolving forces

  1. A force F at angle θ to the ground can be resolved parallel and perpendicular to the ground

  2. Using Pythagoras' Rule, the two components are calculated

  
• 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
• Springs


  To stretch, bend or compress an object, more than one force has to be applied
• Elastic deformation


  The object returns to its original shape when the load has been removed
• Plastic deformation


  The object does not return to its original shape when the load has been removed
• Hooke's Law

  The extension of an elastic object, such as a spring, is directly proportional to the force applied, provided that the limit of proportionality is not exceeded
• Force/Extension Graph

  • Linear line in the elastic region following Hooke's Law, with gradient k
  • Non-linear line in the plastic region not following Hooke's Law

  
• Moment of a force

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

  Can change speed, force or direction by rotation
• The second gear will always turn in the opposite direction to the first gear
• To increase the power, a larger gear is used for the secondary gear as the force on the larger gear is further from its pivot, so the momentum is greater
• Pressure


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


  The upwards force that counteracts the weight of a floating object, equal to the weight of the fluid displaced
• 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