materials

• What is the equation for Hooke's law?

  Force = spring constant x extension

  
  

  F = kx
• What is the spring constant?

  stiffness of the spring (k)
• Stress-strain graph

  Gradient = Young's Modulus

  
• define tough

  Can withstand high impact forces.

  
  

  Absorbs a lot of energy before breaking.

  
  

  Undergoes considerable plastic deformation

  
  

  e.g. concrete (roads)
• Define hard


  Not readily scratched or indented. Resist plastic deformation

  
  

  e.g. diamond (used to remove cataracts)
• Define ductile

  can be drawn into wires

  
  

  show plastic deformation before failure under tension

  
  

  e.g. copper (electrical conductor)
• Define malleable


  Able to be easily bent or flattened, especially into a thin sheet, large plastic deformation under compression

  
  

  e.g. gold (conducts electricity)
• Define brittle

  Undergoes little/no plastic deformation before breaking.

  
  

  Cracks/shatters suddenly (soon after elastic limit reached)

  
  

  little strain for high stress

  
  

  e.g. glass (windows)
• how would you measure the young modulus of a material?

  - copper wire (3m)

  - attach to one end with

  wooden blocks and a g-

  clamp

  - pulley on other end

  - add known masses to the

  wire

  - measure extension using a

  ruler

  - measure diameter using a

  micrometer screw gauge

  - use stress and strain

  equations

  - plug values into YM equation
• young modulus units


  Pa
• Young modulus equation


  stress/strain
• define compressive strain

  causes compression
• define compressive stress


  Causes a decrease in length along the direction of force application (compression)
• Define tensile strain

  The extension produced per unit length
• Define tensile stress

  Causes an increase in length along the direction of force application
• Define breaking stress


  Breaking stress is the maximum force that can be applied on a cross sectional area of a material in such a way that the material is unable to withstand any additional amount of stress before breaking
• strain units

  no units
• Strain equation

  Extension / original length
• Units for stress

  Nm-2 or Pa
• equation for stress

  stress = force/ cross sectional area
• how to investigate Hooke's law?


  - set up clamp stand attach to

  table with g clamp

  - attach spring to clamp

  - add known mass

  - repeat for each mass 3x

  - measure the length of

  extension with a ruler

  - plot a graph of force vs extension
• define plastic behaviour

  material has exceeded its elastic limit and it is permanently deformed

  
  

  small stress will result in a large strain
• define elastic behaviour


  obeys hooke's law and returns to its original length when the force is removed
• Define yield point

  elastic limit of tissue
• define elastic limit


  Point beyond which a wire is permanently stretched
• define the limit of proportionality

  the point at which enough force is applied so that the spring no longer obeys Hooke's law
• force-extension graph

  
• when does a material undergo plastic deformation?

  if force is continuously added the material will stop extended and will no longer return to its original shape
• what can you find when you calculate the area under a force extension graph?

  the stored elastic potential energy
• Define Hooke's Law

  The extension of a spring is directly proportional to the force applied