4.2 - Materials

Question 1

What is Hooke’s Law?

Answer 1

Extension is directly proportional to the Force applied, given that the environmental conditions are kept constant
F=kL
K is the stiffness constant in Nm^-1

Question 2

What equation us used to calculate density ?

Answer 2

Density = mass/volume

kgm^-3

Question 3

What is meant by tensile stress?

Answer 3

The force applied per unit cross sectional area
Stress =force/ CSA
Nm^-2

Question 4

What is tensile strain ?

Answer 4

A measure of how the material stretches : extension divided by original length

Question 5

What is the difference between elastic and plastic deformation ?

Answer 5

Elastic : when force is removed object will return to original size and shape

Plastic : after load is removed the object will not return to original length

Question 6

What is breaking stress?

Answer 6

The minimum stress to break a material

Question 7

What is meant when a material is described as brittle?

Answer 7

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

Question 8

What is the elastic limit?

Answer 8

The force above which the material will be plastically deformed

Question 9

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

Answer 9

The work done to the deformed material

Work done= 1/2 x F x change in length

Question 10

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

Answer 10

E = 1/2 x K x change in length squared

Question 11

What is Young’s Modulus?

Answer 11

Young’s Modulus (E) = tensile stress / tensile strain

It describes the stiffness of a material

Question 12

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

Answer 12

Gradient of the line

Question 13

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

Answer 13

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

Question 14

What is the limit of proportionality and what does it look like on a force extension graph?

Answer 14

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

Question 15

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

Answer 15

Elastic strain energy

Question 16

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

Answer 16

The stiffness constant (k) has not changed, the forces between the atoms are the same when loading and unloading

Question 17

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

Answer 17

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

Question 18

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

Answer 18

1) crumple zones deform plastically in a crash using the cars kinetic energy so less is transferred to the passengers
2) Seat belts stretch to convert the passengers kinetic energy into elastic strain energy

Question 19

Outline the energy changes of a fixed spring from the top is pulled down from the top and released?

Answer 19

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

Question 20

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

Answer 20

Material

Question 21

Where would you find the ultimate tensile stress on a stress-strain graph?

Answer 21

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

Question 22

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

Answer 22

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