Materials

Question 1

Hookes law

Answer 1

Extension is directly proportional to force applied, given that conditions are kept constant
(F=kl)

Question 2

Density equation

Answer 2

Density = mass / volume

Question 3

What is meant by tensile stress

Answer 3

The force applied per unit cross sectional area
Stress = force / CSA

Question 4

What is meant by tensile strain

Answer 4

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

Question 5

Difference between plastic and elastic deformation

Answer 5

Plastic : after the load is removed the object will not return to its original shape
Elastic : when the force is removed the object will return to original shape

Question 6

Breaking stress

Answer 6

The minimum stress needed to break a material

Question 7

Brittle

Answer 7

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

Question 8

Elastic limit

Answer 8

The force above which the material will be plastically deformed

Question 9

Elastic limit

Answer 9

The force above which the material will be plastically deformed

Question 10

Area underneath a force

Answer 10

The work done to deform a material
Work done = 1/2 x F x L

Question 11

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

Answer 11

E = 1/2 kL^2

Question 12

Young’s Modulus

Answer 12

tensile stress / tensile strain
E = FL / LA
Describes stiffness

Question 13

How to find Young’s Modulus from a stress-strain graph

Answer 13

the gradient of the line

Question 14

Graphically represent plastic deformation

Answer 14

Unloading line does not go through origin as the material is plastically derformed

Question 15

How can a force-extension graph show Hookes law is being obeyed

Answer 15

When it is a straight line through the origin

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 Hookes 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 hasnt changed, the forces between the atoms are the same when loading and unloading

Question 19

Why isnt all work done stored as elastic strain energy when a stretch is plastic

Answer 19

Work us 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 cars kinetic energy so less is transferred to the passengers
¬ Seat belts stretch to convert the passengers 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 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

Where would you find the ultimate tensile stress a material can withstand

Answer 22

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