topic 9 - forces and their effects

• scalar quantities : only have magnitude

  vector quantities : have both magnitude and direction
• examples of scalar quantities

  • distance
  • mass
  • temperature
  • density
  • energy
  • speed
  • time
• example of vector quantities

  • weight
  • velocity
  • acceleration
  • force
  • displacement
• Efficiency

  • efficiency = useful energy / total energy
  • efficiency= useful power / total power
• Friction is a force that is always against a motion between surfaces.
• Drag is a force that is always against a motion through a fluid.
• Thrust is a forward force or movement given to an object due to the expulsion of an object.
• Tension is a force through a material opposing an action force.
• Upthrust is a force acting up due to the displacement of a fluid.
• Electromagnetic force is an attraction or repulsion depending on the charge or poles.
• Gravitational force, also known as weight, is a force exerted on another object due to a massive object.
• Normal contact force is a reaction force or force that opposes a force applied on a surface.
• Spring force is a force acting on a spring either to expand or compress.
• lubricants

  • reduces amount of friction ( wasteful energy )
  • reduces amount of work done against friction
• force : is a push or pull that acts on an object , causing a change in motion onto an object ( acceleration )
• resultant : the sum of all vectors of the same type , resulting in a single vector acting on an object

  resultant force : sum of all individual forces acting on an object , resulting in a single force causing acceleration
• objects affecting each other

  • objects interact by exerting forces on eachother ,
  • these two forces ( earth and moon example ) are action reaction forces ( newton's third law pair ) . these forces are the same size , opposite direction and acting on different objects
  • no actual contact between moon and earth - fields ( electric and gravitational )
  • these are non contact forces
• types of forces

  1. contact forces

  • tension
  • friction
  • upthrust
• types of forces

  1. contact forces

  • tension
  • friction
  • upthrust

  2. non contact forces

  • gravity
  • static electricity - magnetism
• examples of contact forces

  • upthrust/lift - act upwards in air
  • friction (on land ) / air resistance (in air ) / drag ( in water ) - act against the direction of movement
  • push/pull - forces that also act in the direction of movement , generated by direct contact between objects ( not by engine power )
• examples of non contact force

  • weight ( gravity ) - acts between objects with mass
  • magnetic force - magnets can attract objects from magnetic materials , can attract or repel objects , do not have to physically touch to affect each other
  • electric field - object charged with static electricity , has electric fields around it , can affect objects
• contact forces - between two objects touching eachother
• non-contact force = occurs between two objects due to a field : without touching
• field : a region of space in which a object will experience a force based on one of its properties
• types of fields - magnetic fields

  • magnetic force
  • non-contact
  • can be attractive or repulsive
• types of fields - electrical fields

  • electrostatic force
  • non - contact
  • can be attractive / repulsive
• types of field - gravitational field

  • gravitational force ( weight )
  • non - contact force
  • always attractive
• free body diagrams

  1. forces originate from a body
  2. forces are acting from a single object
  3. forces must be labeled
  4. arrows must be proportional to size of force
• weight must always act downwards , towards the centre of a massive object
• the normal contact force must be perpendicular to the surface
• vectors :

  • vectors which are perpendicular to each other are independent of one another - cannot be added or taken away
  • conventionally up is positive and towards the right is positive
• vector diagrams

  • resulting ( vectors ) : representing a single force as two-forces drawing the horizontal and vertical component
• reducing friction

  • using a lubricant : devices waste a lot f energy for various reasons due to friction between moving parts
  • energy is wasted in the form of : thermal and sound
• thermal energy : the part around , where energy is wasted due to friction, will increase in temperature as they gain thermal energy
• benefits of reducing friction :

  • it is the amount of friction is reduced , less energy is wasted
  • the efficiency of the device increases
• using lubricants

  • a method of reducing the amount of energy wasted , due to friction is by using a lubricant
  • lubricant reduces the friction experienced around a turning point of a device by allowing the surface to slide smoothly over eachother
  • examples : door hinge , car engine ( moving pistons ) , gears
• moments

  • a moment is the turning effect of a force around a fixed point ( called a pivot )
  • the size of a moment depends on two factors
  • size of the force applied
  • perpendicular distance from the pivot to the line of action of the force
• moment of a force ( Nm / newton metres ) = Force ( N ) x distance perpendicular to direction of force ( m )
• principle of moments

  sum of clockwise moments = sum of anti-clockwise moments

  when in equilbrium
• leavers

  • leavers are used to transfer force
  • reduce force required to perform an action

  A) effort distance

  B) effort

  C) load distance

  D) load