11.3 Deformation of solids

Question 1

What is needed to stretch or twist an object

Answer 1

A pair of forces

Question 2

What is the elasticity of a solid material

Answer 2

Its ability to regain its shape after it has been deformed or distorted and the forces that deformed it have been released

Question 3

Deformation that stretches an object is…

Answer 3

…tensile

Question 4

Deformation that compresses an object is…

Answer 4

…compressive

Question 5

How to test materials to see how easily they stretch

Answer 5

Material is held at its upper end and loaded by hanging weights at its lower end
A set square or pointer attached to the bottom of the weights may be used to measure the extension of the material, as the weight of the load is increased in steps then decreased to zero. Tension of the material is equal to its weight

Question 6

What is the extension of a material

Answer 6

Its increase of length from its unloaded length

Question 7

What sort of graph does the extension of a steel spring show

Answer 7

A straight line in accordance with Hooke’s law

Question 8

Stretching a rubber band

Answer 8

At first extends easily when it is stretched, however it becomes fully stretched and very difficult to stretch further when it has been lengthened considerably

Question 9

Stretching a polythene strip

Answer 9

Gives and stretches easily after its initial stiffness is overcome.
However, after giving easily, it extends little and becomes difficult to stretch

Question 10

What setup is used to measure the extension of a wire under tension

Answer 10

Searle’s apparatus
Micrometer attached to the control wire is adjusted so the spirit level between the control and test wire is horizontal. Test wire is loaded causing it to extend slightly meaning that the spirit level to drops on one side. Micrometer readjusted to make spirit level horizontal again.

Question 11

For a wire of length L and area of cross section A under tension, the tensile strength of the wire δ

Answer 11

δ=T/A

Where:
T - tension
A - area of cross section under tension

Question 12

What is the unit of stress

Answer 12

Pascal
Pa
Equal to 1Nm^-2

Question 13

For a wire of length L and area of cross section A under tension, the tensile strain in the wire ε

Answer 13

ε = ΔL/L

Where ΔL is the extension of the wire

Question 14

What is the unit of strain

Answer 14

No unit as it is a ratio

Question 15

What to remember about measuring the extension

Answer 15

Always measured from the original unstratched length of the object

Question 16

From 0 to the limit of proportionality P, the tensile stress is ______ to the tensile strain

Answer 16

Proportional

Question 17

What is the Young modulus of a material

Answer 17

The value of stress/strain which is a constant

Question 18

Young modulus E =

Answer 18

tensile strength/tensile strain = TL/AΔL

Question 19

What happens beyond the limit of proportionality P

Answer 19

The line curves and continues beyond the elastic limit E to the yield point Y1 which is where the wire weakens temporarily.

Question 20

What is the elastic limit

Answer 20

The point beyond which the wire is permanently stretched and suffers plastic deformation

Question 21

What is beyond maximum tensile stress

Answer 21

The ultimate tensile stress

Question 22

What happens at the ultimate tensile stress

Answer 22

The wire loses its strength, extends, and becomes narrower at its weakest point

Question 23

What causes the increase of tensile stress

Answer 23

Due to the reduced area of cross section at this point until the wire breaks

Question 24

What is the ultimate tensile stress also referred to as

Answer 24

The breaking stress

Question 25

How can the stiffness of different materials be copared

Answer 25

By using the gradient of the stress-strain line which is equal to the Young modulus of the material

Question 26

What is the strength of a material

Answer 26

Its ultimate tensile stress

Which is its maximum tensile stress

Question 27

Why is steel stronger than copper

Answer 27

Its maximum tensile stress is greater

Question 28

A brittle material ______ without any ______ ______.

Answer 28

Snaps

Noticeable yield

Question 29

A ductile material can be ______ into a ______.

Answer 29

Drawn

Wire