6 : Materials

Question 1

hooke’s law

Answer 1

• extension is proportional force
• f =kx
• k is force constant (N m^ -1) that depends on the object being stretched = stiffness

Question 2

Type of forces

Answer 2

• tensile forces = extension
• compressive forces - compression

Question 3

hooke’s law graph

Answer 3

• straight-line relationship between force and extension (linear)
• when force becomes great enough the graph starts to curve beyond the limit of proportionality
• then there is the elastic limit where the material is permanently stretched when the force is removed the material no longer goes back to original shape

Question 4

springs in parallel

Answer 4

k = k1+k2

Question 5

springs in series

Answer 5

1/k = 1/k1 + 1/k2

Question 6

elastic deformation

Answer 6

• material is under tension the atoms are pulled apart from one another
• move slightly from equilibrium positions without changing position in the material
• oce load is removed atoms return to their equilibrium distance apart

Question 7

plastic deformation

Answer 7

• some atoms in the material move position relative to each other
• when load is removed the atoms don’t return to equilibrium position

Question 8

tensile stress

Answer 8

• the force applied divided by the cross-sectional area
• Nm^-2 or Pa

Question 9

tensile strain

Answer 9

• the change in length
• the extension divided by original length
• no units

Question 10

Ultimate tensile strength

Answer 10

the maximum strength a material can withstand

Question 11

Breaking point

Answer 11

• the stress becomes so great that atoms are completely separated and material breaks

Question 12

Elastic potential energy

Answer 12

• Force-extension graph - work done is area under graph
• before elastic limit all the work done is stored as potential energy
• 1/2Fx

Question 13

energy stored

Answer 13

• 1/2kx^ 2

Question 14

youngs modulus

Answer 14

• up to limit of proportionality stress and strain are proportional
• below limit stress/ strain = the constant young’s modulus
• Nm^-2
• measure of stiffness of a material

Question 15

stress-strain graph

Answer 15

• gradient = young’s modulus
  area under gives elastic potential energy stored per unit volume

Question 16

yield point

Answer 16

• where the material begins to stretch without any extra stress a large amount of plastic deformation

Question 17

brittle fracture

Answer 17

• when stress reaches a certain point a material snaps
• a brittle material - tiny cracks at the materials surface gets bigger and bigger until material breaks completely

Question 18

different materials

Answer 18

• the stronger the material the higher the breaking stress
• stiff materials have higher young’s modulus as arde difficult to stretch or compress
  -for a given stress a stiff material will have lower strain
• some stiff materials break under low stress

Question 19

polythene example

Answer 19

• molecules that makeup polymeric materials are arranged in long chains
• behaves plastically
• a ductile material

Question 20

Rubber example

Answer 20

• return to original length when load is removed
• loading and unloading are different
• the energy released hen rubber is unloaded is less than the work done to stretch rubber
• some of elastic potential energy is converted into heat = the area between curves