forces

• Contact force


  Force that requires two objects to be touching for it to act
• Non-contact force


  Force that does not require the two objects to be touching for it to act
• Examples of contact forces

  • Friction
  • Air resistance
  • Tension in ropes
  • Normal contact force
• Examples of non-contact forces

  • Magnetic force
  • Gravitational force
  • Electrostatic force
• When two objects interact, there is a force produced on both objects
• Interaction pairs


  • Sun and Earth are attracted to each other by the gravitational force
  • Chair and ground push on each other with the normal contact force
• Gravitational force


  The force of attraction between masses
• Gravity attracts all masses, but you only notice it when one of the masses is really really big, e.g. a planet
• Mass


  The amount of 'stuff' in an object, which has the same value anywhere in the universe
• Weight


  The force acting on an object due to gravity (the pull of the gravitational force on the object)
• Gravitational field strength varies with location, being stronger the closer you are to the mass causing the field, and stronger for larger masses
• The weight of an object depends on the strength of the gravitational field at the location of the object, so it changes with location
• Weight is a force measured in newtons, acting from the centre of mass of the object
• Mass is measured in kilograms using a mass balance
• Direct proportionality


  Weight and mass are directly proportional, so if mass doubles, weight doubles
• Calculating weight


  • Weight (N) = Mass (kg) x Gravitational Field Strength (N/kg)
  • On Earth, g=9.8 N/kg, on the Moon g=1.6 N/kg
• Resultant force


  The single force that has the same effect as all the original forces together
• Work


  Energy transferred when a force moves an object through a distance
• Work done (J) = Force (N) x Distance (moved along the line of action of the force)
• Using scale diagrams to find resultant forces

  1. Draw all the forces acting on an object, to scale, 'tip-to-tail'

  2. Draw a straight line from the start of the first force to the end of the last force - this is the resultant force

  3. Measure the length and angle of the resultant force to find its magnitude and direction
• Applying a force to an object

  1. May cause it to stretch, compress or bend

  2. Requires more than one force acting on the object
• Work


  Done when a force stretches or compresses an object and causes energy to be transferred to the plastic potential energy store of the object
• Elastic deformation


  Object can go back to its original shape and length after the force has been removed
• Inelastic deformation


  Object doesn't return to its original shape and length after the force has been removed
• Extension of a stretched spring (or certain other elastic objects)

  Directly proportional to the force applied
• Spring constant


  Depends on the material that you are stretching - a stiffer spring has a greater spring constant
• Force


  Directly proportional to extension (F=ke)
• Limit of proportionality


  The maximum force above which the graph of force against extension curves, showing that extension is no longer proportional to force
• Moment of a force

  The turning effect of a force
• Levers


  • Increase the distance from the pivot at which the force is applied, meaning less force is needed to get the same moment, making it easier to do work
• Gears


  Circular discs with 'teeth' around their edges that interlock to transmit the rotational effect of a force from one place to another
• Different sized gears


  Can be used to change the moment of the force - a force transmitted to a larger gear will cause a bigger moment, as the distance to the pivot is greater, but the larger gear will turn slower than the smaller gear
• Vector quantities have a magnitude and a direction. It includes velocity, displacement, force, and acceleration.
• Scalar quantities only have magnitude, no direction. It includes speed, distance, mass, time, temperature, and energy.
• Fluids


  • Substances that can 'flow' because their particles are able to move around
  • Particles collide with surfaces and other particles
• Calculating pressure in a fluid

  Pressure (Pa) = Force (N) / Area (m²)
• Pressure in a Liquid


  • Depends on Depth and Density
• Density


  Measure of the 'compactness' of a substance, i.e. how close together the particles in a substance are
• For a given liquid, the density is uniform (the same everywhere) and it doesn't vary with shape or size
• The more dense a given liquid is, the more particles it has in a certain space, so the pressure is higher