Section 4 - Mechanics and Materials: 4.2 - Materials

Question 1

What is Hooke’s law?

Answer 1

Extension (∆L) is directly proportional to force
applied (F), given that the environmental
conditions are kept constant.

F= k∆L k is the stiffness constant in Nm^-1

Question 2

What equation is used to calculate density?

Answer 2

Density = Mass / Volume
Density units: kgm^-3
Mass: kg
Volume: m^3

Question 3

What is meant by tensile stress?

Answer 3

The force applied per unit cross sectional area.
Stress = force / CSA
Stress units: Nm-2
Force units: N
Cross sectional area units: m2

Question 4

What is tensile strain?

Answer 4

A measure of how the material stretches: the
extension (ΔL) divided by the original length (L),
strain has no units.
Strain = ΔL / L

Question 5

What is the difference between elastic and plastic
deformation?

Answer 5

Elastic deformation: when the force is removed
the object will return to its original shape.

Plastic deformation: after the load is removed the
object will not return to its original shape.

Question 6

What is breaking stress?

Answer 6

The minimum stress needed to break a material.

Question 7

Which of these two graphs represents a brittle
material?
(Insert Graphs A and B at home)

Answer 7

Insert Graph A

Question 8

What is meant when a material is

described as brittle?

Answer 8

It doesn’t deform plastically but breaks when the
stress reaches a certain value.

Question 9

What is the elastic limit?

Answer 9

The force above which the material will be
plastically deformed (permanently stretched).

Question 10

What does the area underneath a force - extension
graph represent?

Answer 10

The work done to deform the material.

Work done= 1⁄2 x F x ΔL

Question 11

State the equation to calculate elastic strain energy
from the spring constant and extension.

Answer 11

E = 1⁄2 kΔL2

Question 12

What is Young’s modulus?

Answer 12

Young’s modulus (E) = tensile stress/ tensile strain.

E = FL / ΔLA

(by substituting stress and strain equations).

It describes the stiffness of a material.

Question 13

How do you find the Young’s modulus from a
stress-strain graph?

Answer 13

The gradient of the line.

Question 14

Which of these graphs would represent a wire which
has plastically deformed?
(Insert pictures of graphs and key)

Answer 14

The unloading
line doesn’t go
through the
origin as the
material is
permanently
extended
(stretched)
(Instert picture of right graph with key)

Question 15

How can a force-extension graph show Hooke’s Law
is being obeyed?

Answer 15

When it is a straight line through the
origin ie. force and extension are directly
proportional.

Question 16

What is the limit of proportionality and

what does it look like on a
force-extension graph?

Answer 16

The point after which Hooke’s law is no
longer obeyed, it is shown by the line
beginning to curve on a force-extension
graph.

Question 17

How is the work done to stretch or compress a
material stored?

Answer 17

Elastic strain energy.

Question 18

Why are the loading and unloading lines parallel on
a force-extension graph for a plastically deformed
material?

Answer 18

The stiffness constant (k) hasn’t
changed, the forces between the atoms
are the same when loading and
unloading.

Question 19

Why isn’t all work done stored as elastic strain
energy when a stretch is plastic?

Answer 19

Work is done to move atoms apart, so
energy is not stored as elastic strain
energy but is dissipated as heat.

Question 20

How is the dissipation of energy in plastic
deformation used to design safer vehicles?

Answer 20

Crumple zones deform plastically in a crash using
the car’s kinetic energy so less is transferred to
the passengers.
● seat belts stretch to convert the passenger’s
kinetic energy into elastic strain energy.

Question 21

Outline the energy changes that occur when a spring
fixed at the top is pulled down and released

Answer 21

The work done in pulling the spring down
(stretching it) is stored as elastic strain energy,
when the spring is released this is converted to
kinetic energy which is converted to gravitational
potential energy as the spring rises

Question 22

Do stress-strain graphs show the
behaviour of a material or a specific

object?

Answer 22

Material.

Question 23

Where would you find the ultimate tensile

stress on a stress-strain graph?

Answer 23

The highest point on a graph, it is the
maximum stress a material can
withstand.

Question 24

What would the stress-strain graph for a ductile
material look like?

Answer 24

A ductile material can undergo a large amount of
plastic deformation before fracturing.

(Inset Graph)