Topic 9: Forces and their Effects

• Non-contact forces act between 2 objects not physically touching each other:

  • Magnetic - experiences by magnetic material in a magnetic field where opposite poles attract and same ones repel
  • Electrostatic - experienced by charged particles in an electric field where opposite charges attract and same ones repel
  • Gravitational - experiences by mass in a gravitational field where masses are attracted towards each other
• Contact forces act between 2 objects physically touching each other:

  • Normal Contact - object resting on surface experiences a reaction force perpendicular to the surface
  • Tension - when an object is stretched
  • Friction - when 2 objects slide past each other
  • Air Resistance - when an object moves through the air
• Newton's Third Law of Motion - for every action there is an equal and opposite reaction
• Vector:

  • has size and direction
  • displacement
  • velocity
• Scalar:

  • has only size
  • distance
  • speed
• Two forces can be added to find a resultant force.

  Vector diagrams can resolve forces into a horizontal component acting to the right and a vertical component acting upwards:

  • right-angled triangle where each side represents a force
  • measure unknown lengths to find resolved forces
  • two components equal the hypotenuse

  
• Free Body Diagram:

  • object represented by a box/dot
  • forces shown through arrows
  • label arrows in force type and magnitude

  
• When 2 or more forces act on an object, the resultant force can be found by finding the difference between 2 opposing forces:

  • Box on a Table - weight of box (acting down) is 50N and normal reaction force (acting up) is 50N, the forces are balanced so resultant force is 0N
  • Falling Box - box weight is 50N and air resistance (acting up) is 20N, the forces are unbalanced so resultant force is 30N downwards
• Moment - the turning effect of a force around a fixed point called a pivot

  Principle of Moments - for a balanced system in equilibrium, the sum of clockwise moments must be balanced by the sum of anti-clockwise moments
• A lever consists of a pivot, an effort and a load.

  They transfer the turning effect of a force and use moments to act as a force multiplier which makes it easier to do work
• Levers increase the distance from the pivot at which force is applied so the longer the lever, the smaller the force needed to give the same moment

  
• Gears are circular cogs with 'teeth' around the edge that can interlock so that turning one causes another to turn in the opposite direction - transmit rotational effect of forces
• A force applied to a small gear creates a small moment that applies to the next gear.

  If it's a large gear then force is applied further from its pivot so moment is larger
• Interlocked gears will rotate at different speeds - the larger the gear the slower it spins
• Lubrication increases the efficiency of gears as it reduces friction and unwanted energy transfers (dissipation).

  Adding an oily or greasy liquid holds the surfaces slightly apart so they can slide past each other with less friction