Forces

• 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
• Scalar quantities


  • Speed
  • Distance
  • Time
  • Mass
  • Energy
• Vector quantities


  • Velocity
  • Displacement
  • Acceleration
  • Force
  • Momentum
• Displacement is 0 at the height of a cliff, positive above the cliff, and negative below the cliff
• In long answer questions, you may be able to decide where the "0" point of a vector may lie
• Velocity


  Speed with a given direction
• A car travelling round a roundabout at constant speed has a constantly changing velocity, therefore it is accelerating
• Force


  A push or pull that acts on an object due to the interaction with another object
• Types of forces


  • Non-contact forces (electrostatic, gravitational attraction)
  • Contact forces (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, only weight acts

  2. As he falls, air resistance increases, decreasing acceleration

  3. Eventually air resistance equals weight, so no resultant force and he reaches terminal velocity
• Free body diagram


  Shows 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' Theorem

  
• Work


  Energy transferred from the object doing the work to another form
• Work done against frictional forces causes a rise in temperature of the object
• To stretch, bend or compress an object, more than one force has to be applied
• Deformation


  Changing the shape of an object
• 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 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
• 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 is applied at a further distance from the pivot
• 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
• 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 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