Materials

Question 1

What are forces that produce extension known as

Answer 1

Tensile forces

Question 2

What are forces that compress an object known as

Answer 2

Compressive forces

Question 3

What does a helical spring undergo when tensile forces are exerted

Answer 3

Tensile deformation

Question 4

What does a helical spring undergo when compressive forces are exerted

Answer 4

Compressive deformation

Question 5

Describe a force extension graph

Answer 5

Straight line from the origin to elastic limit of the spring - in this linear region, spring undergoes elastic deformation - it will return to its original length when the force is removed

Beyond this point - it will go plastic deformation - permanent structural changes occur and the spring won’t go back to its original length when the spring is removed

Question 6

What is Hooke’s Law

Answer 6

Extension of the print is direct proportional to the force applied as long as the elastic limit of the spring is not exceeded

Question 7

Hookes Law

Answer 7

F = kx

K = force constant (Nm^-1)

Question 8

What is a force constant

Answer 8

Measure of stiffness of a spring

Larger force constant means it’s more difficult to extnemt

Question 9

Where can Hookes Law also be applied to

Answer 9

Wires under tension

Concrete columns under compression

Question 10

What happens to work done when a material has gone though plastic deformation and when it hasn’t

Answer 10

Some of the work done has gone to moving the atoms into new positions - is not recoverable

When it hasn’t - work done can be fully recovered

Question 11

Work done by a force in extending the spring equation

Answer 11

Change in work done = force x change in extension

Question 12

What is the area under a force extension graph

Answer 12

Work done

Question 13

What is work done on a spring transferred into and is it recoverable

Answer 13

EPE

Yes because if the elastic behaviour of the spring

Question 14

Equation for EPE

Answer 14

E = 1/2kx^2

For a spring obeying Hooke’s Law substitute into 1/2Fx (Area under graph)

Question 15

What is the hysteresis loop

Answer 15

The loop formed by loading and unloading curves for objects that don’t obey Hooke’s Law

Question 16

When is more work done - stretching a rubber band or decreasing it

Answer 16

Stretching a rubber band

Question 17

What is the area inside the hysteresis loop

Answer 17

Thermal energy released when a material is loaded then unloaded

Question 18

What is tensile strsss

Answer 18

Force applied per unit cross sectional area of the wire

Question 19

Tensile stress formula

Answer 19

T.S = Force / cross sectional area

(Sigma is sign for T.S)

Question 20

Tensile strain formula

Answer 20

Fractional change in the original length of the wire

Question 21

Tensile strain formula

Answer 21

Extension / original length

T.S is represented with epsilon

Question 22

What is a ductile material

Answer 22

A material that can easily be drawn into a wire or hammered into thin sheets

Question 23

Describe a stress strain graph for a metal wire

Answer 23

Stress is directly to strain from origin to the limit of proportionality - obeys Hooke’s law in this region

Small curve to E - represents the Elastic
limit - some materials obey Hooke’s law to this limit - elastic deformation occurs beyond this limit

Curves up to Y1 - upper yield point
Curves down to Y1 - Lower yield point

Yield point is where the material extends rapidly - yield points may be absent in some materials - typical for mild steel

Y2 to UTS - Ultimate Tensile Strength - maximum stress a material can go under before breaking

Question 24

What do strong materials have

Answer 24

High UTS

Question 25

What is Young’s Modulus

Answer 25

Ratio of stress to strain for material within the limit of proportionality

Question 26

Youngs Modulus Formula

Answer 26

YM = Tensile Stress / Tensile Strain Nm^-2 or Pa Fl/Ax

Question 27

Determine Youngs Modulus

Answer 27

Measure diameter of a wire with a micrometer Calculate cross sectional area with this measurement Repeat and average measurements from several places along the wire for more accurate reading Tensile force acting on wire can be calculated with F=mg (m is hanging Mass) After applying each additional mass - extension calculated is (extended length - original length) - repeat these for 6 different masses Plot a stress and strain graph after calculating them for each load Youngs Modulus can be calculated up from the linear section of the graph

Question 28

Stress Strain graph for brittle materials

Answer 28

Elastic behaviour up to its breaking point with plastic deformation (just straight lines both ways)

Question 29

What are polymeric materials

Answer 29

Materials with long molecular chains

Question 30

Stress strain graph for polymers

Answer 30

Dependent on material Some show elastic behaviour and others show plastic behaviour

Question 31

What is force directly proportionally to and when

Answer 31

Extension For a spring obeying Hooke's Law

Question 32

What is the gradient of a force-extension grant

Answer 32

The force constant

Question 33

Where does Hooke's Law apply to besides springs

Answer 33

Wires under tension and concrete columns under compression Any object that can be elastically squashed or extended

Question 34

Loading curve (force-extension graphs) for a metal wire

Answer 34

Follows Hooke's Law until the elastic limit of the wire Unloading graph will be identical for forces less than the elastic limit Beyond the elastic limit it is parallel to the loading graph but not identical to it - wire is permanently extended after the force is removed - longer than it was at the start

Question 35

Loading curves (force-extension graph) for a rubber band

Answer 35

Do not obey Hooke's Law - will return to its original length once Hooke's Law is removed Loafing and unloading curves are different - a loop is formed between them Thermal energy released is area inside the hysteresis loop

Question 36

Loading curves (force-extension graph) for polythene

Answer 36

Do not obeys Hooke's Law - suffer plastic deformation under a relatively little force

Question 37

What is the breaking strength of a material

Answer 37

The stress value at the point of fracture