Materials

Question 1

What is Hooke’s law?

Answer 1

Extension is directly proportion to force applied, given that the environmental condition are kept the same.

Question 2

What equation is used to calculate density

Answer 2

Density = mass / volume

Question 3

What is meant by tensile stress?

Answer 3

The force applied per cross sectional area

Question 4

What is tensile strain?

Answer 4

A measure of how the material stretches: the extension divided by the original length. Strain how NO units

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

What is meant when a material is described as brittle?

Answer 7

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

Question 8

What is the elastic limit?

Answer 8

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

Question 9

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

Answer 9

The work done to deform the material.

Question 10

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

Answer 10

E = 1/2 K /\ L^2

Question 11

What is Young’s modulus?

Answer 11

Young’s modulus = tensile stress/ tensile strain.

it describes the stiffness of a material.

Question 12

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

Answer 12

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

Question 15

How is the work done to stretch or compress material stored

Answer 15

Elastic strain energy

Question 16

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

Answer 16

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

Question 17

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

Answer 17

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

Question 18

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

Answer 18

• 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 19

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

Answer 19

The work done in pulling the spring down 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 20

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

Answer 20

Material

Question 21

Where would you find the ultimate tensile stress on an 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.