4.2 Materials

Question 1

Define density.

Answer 1

Mass per unit volume

Question 2

What is Hooke’s law?

Answer 2

Extension is directly proportional to force applied

Question 3

Define tensile strain.

Answer 3

The ratio of an object’s extension to its original length.

Question 4

Define tensile stress.

Answer 4

Force applied per unit cross sectional area

Question 5

What is the equation for Elastic Potential Energy

aka Elastic strain energy

Answer 5

Question 6

What is the area of a force-extension graph?

Answer 6

Energy stored
or
Elastic Potential Energy

Question 7

Define breaking stress.

Answer 7

Maximum stress an object can withstand before failure.

Question 8

Define brittle.

Answer 8

A material that will not plastically deform by allowing cracks to propagate through it.

Having little elasticity.

Question 9

Define tough.

Answer 9

A material that can absorb energy before breaking and is resistant to the propagation of cracks.

Ability to deform plastically/permanently.

Question 10

Define ductile.

Answer 10

A material can be drawn into wires without losing strength. Or just have their shape
changed/deformed without losing strength.

Question 11

Define malleable.

Answer 11

A material that may lose strength when its shape is deformed.

Question 12

Define hard.

Answer 12

A material that is very resistant to cutting, indentation, stretching, and abrasion.

Cutting tools have to be harder than the material they are cutting.

Question 13

Define stiff.

Answer 13

A material that is resistant to bending and stretching/resistant to deformation under the action of an applied force.

Measured by Youngs modulus. Higher Youngs modulus, the stiffer it is.

Question 14

Define strength.

Answer 14

A material that is resistant to breaking under the strain of an applied force/the stress required to break a material.

Question 15

Define fracture stress.

Answer 15

Is the level of stress at which a material will fracture.

Question 16

Define elastic limit.

aka yield strength or yield stress

Answer 16

Is the level of stress at which a material will deform permanently.

Question 17

Define elastic deformation.

Answer 17

A material will return to its original shape once the forces acting on it have been
removed.

Question 18

Define plastic deformation.

Answer 18

A material’s ability to be permanently stretched or deformed.

When force is removed, atoms don’t return to their original positions.

Question 19

Define brittle fracture.

Answer 19

Cracks are
allowed to propagate through the material.

Question 20

What does elastic deformation look like on a force-extension graph?

Answer 20

Question 21

What does plastic deformation look like on a force-extension graph?

Answer 21

Question 22

What does the enclosed area of a plastic deformation graph represent?

Answer 22

Work done to permanently deform the material.

Question 23

Which graph is for a brittle material?

Answer 23

D

Question 24

Which graph is for a ductile material?

Answer 24

B

Question 25

What is the area of a force extension graph?

Answer 25

Elastic strain energy

aka Elastic Potential Energy

Provided material obeys Hooke’s law

Question 26

How is energy conserved in elastic deformation?

Answer 26

Work done is stored as elastic strain energy.

Question 27

How is energy conserved in plastic deformation?

Answer 27

Work is done to separate atoms. Energy is not stored as strain energy.

Mostly dissapated as heat

Question 28

State the energy transfers when a mass on a sping is pulled down and released.

Answer 28

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 29

What is a stress-strain curve and its features?

Answer 29

Question 30

Which curves are brittle, ductile, and plastic?

Answer 30

• Blue is brittle
• Purple is ductile
• Green is plastic

Question 31

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

Answer 31

Crumple zones deform plastically in a crash using the car’s kinetic energy so less is transferred to the passengers.

Question 32

Define Young’s Modulus.

Answer 32

Gradient of a stress-strain graph.