Chapter 12: Properties Of Materials

Question 1

What is density?

Answer 1

The mass per unit volume of a material

ρ=mass/volume
Mass in kg, volume in m^3

Question 2

What is Hooke’s Law in words?

Answer 2

The extension of an elastic object is directly proportional to the force applied to it up to the object’s limit of proportionality
F=ke

Question 3

What is Hooke’s Law?

Answer 3

F=ke
Force in Newtons
Extension in metres
Spring constant in N/m

Question 4

What is a spring constant?

Answer 4

A measure of how hard it is to bend or stretch a spring.
It defines the stiffness of a spring as the force required for a unit extension of the spring (N/m).
A large k means the spring is stiff.

Question 5

What are compressive forces?

Answer 5

Forces that tend to squeeze an object and reduce its size in the direction that the forces are applied.

Question 6

What are tensile forces?

Answer 6

Forces that act to pull or stretch an object.

Question 7

What is extension?

Answer 7

The length a material has stretched when a load is added.
extension= stretched length - original length

Question 8

What is the limit of proportionality?

Answer 8

The endpoint of the linear section of a force extension graph after which extension is no longer proportional to the load.

Question 9

What is the elastic limit?

Answer 9

The force above which a material is permanently deformed (plastically deformed).
… and below which the material will return to its original length when the load is removed.

Question 10

What is elastic deformation?

Answer 10

When a material will return to its original shape once the applied forces are removed

Question 11

What is plastic/inelastic deformation?

Answer 11

When a material does not return to its original shape once the applied forces are removed. It is permanently deformed.

Question 12

What is ductility?

Answer 12

The ability of a material to be formed into wires by stretching them. It is an example of plastic deformation.

Question 13

How does a brittle material act?

Answer 13

Shows little or no plastic deformation before breaking (once the elastic limit is exceeded), eg glass.

Question 14

How do ductile materials fracture?

Answer 14

The material will elongate and ‘neck’ before breaking (plastic region on graph).

Question 15

How does a brittle material fracture?

Answer 15

No change of shape as it does not plastically deform; straight break in the material.

Question 16

What is elastic strain energy?

Answer 16

Energy stored in a material that is equivalent to the work done in stretching the material.

Question 17

How do you calculate elastic strain energy?

Answer 17

Area under a force extension graph

Energy stored= (Fe)/2

Question 18

How does strain energy differ when a rubber band is loaded and unloaded?

Answer 18

Strain energy stored is greater when it’s being loaded than when unloaded (it becomes warmer as it stretches and relaxed).

Question 19

What happens to extension when two springs in connected in series or parallel?

Answer 19

Series: extension doubles
Parallel: extension halves

Question 20

What is tensile stress?

Answer 20

A measurement of the force applied over cross-sectional area of a material.

tensile stress=force/cross-sectional area
σ= F/A
Force in Newtons, Area in m^2, σ in Pa

Question 21

What is tensile strain?

Answer 21

The ratio of extension to original length of a material

tensile strain= extension/original length
Ɛ=x/l
Extension (x) in m, length in m, Ɛ dimensionless

Question 22

What is Young Modulus?

Answer 22

A measure of stiffness of an elastic material and does not depend on the dimensions of the sample being tested. Measured in Pa or N/m^2

Question 23

What is the equation for Young modulus?

Answer 23

Young modulus = stress/ strain
E= σ/Ɛ
Young modulus= Fl/Ax

Question 24

How is the Young Modulus found using a stress strain graph?

Answer 24

Gradient of the straight line part of the graph (up to LoP).

Question 25

What is the ultimate tensile stress/strength of a material?

Answer 25

The greatest stress a material can withstand before breaking.

Question 26

What is creep?

Answer 26

A property some metals have in which extension increases over time even though stress applied is not increased.