Unit 5, Solids Under Stress

Question 1

What is the point at which a spring starts to become deformed

Answer 1

The elastic limit

Question 2

Hooke’s law

Answer 2

The tension in a spring or wire is proportional to its extension from its natural length provided the extension is not too great

Question 3

What is the equation for the spring constant

Answer 3

F = kx

Question 4

how would you calculate work done with a force-extension graph

Answer 4

Calculate the area under the graph

Question 5

What is the equation for work done using Hooke’s law

Answer 5

W = 1/2kx^2

Question 6

What is Young’s modulus used to compare

Answer 6

Different materials of various lengths and cross-sectional areas

Question 7

What is the equation for Young’s modulus in word form

Answer 7

E is equal to Stress divided by Strain

Question 8

Stress

Answer 8

The force per unit cross-sectional area when equal opposing forces act on a body

Question 9

What is the equation for stress

Answer 9

sigma = Force/Area

Question 10

Strain

Answer 10

The extension per unit length due to an applied stress

Question 11

What is the equation for strain

Answer 11

Epsilon = Change in length / original length

Question 12

What are the units of stress

Answer 12

Pascal, Pa

Question 13

What are the units of Strain

Answer 13

There are no units

Question 14

What are the three types of solids

Answer 14

Crystalline
Amorphous
Polymeric

Question 15

Crystalline

Answer 15

Is a solid in which atoms are arranged in a regular array, there is a long rand order within the structure

Question 16

Amorphous

Answer 16

A solid in which there is no regular order, atoms are arranged randomly

Question 17

Polymeric

Answer 17

A solid that consists of chain-like molecules

Question 18

On an annotated stress-strain graph what does the P point stand for

Answer 18

Limit of Proportionality, Where Hooke’s law stops being followed

Question 19

On an annotated stress-strain graph what does the E point stand for

Answer 19

Elastic Limit, beyond this point all extension is plastic strain

Question 20

What is the difference between plastic and elastic strain

Answer 20

in elastic strain the material will return to it’s original point after the force is removed, with plastic strain the material no longer returns to the original point

Question 21

On an annotated stress-strain graph what does the Y point stand for

Answer 21

Yield Point, Beyond this point a small increase in the force applied will cause a larger than before extension

Question 22

On an annotated stress-strain graph what does the X point stand for

Answer 22

Breaking Point, The stress at this point is known as the breaking stress

Question 23

Dislocation

Answer 23

A gap in a plane of atoms where one plane is incomplete

Question 24

How to reduce dislocations and there effects

Answer 24

Introduce impurities into the material

Question 25

What is necking

Answer 25

Local thin`ning due to dislocations, which increases the stress around the area

Question 26

What are the two methods to increasing the breaking stress of brittle materials

Answer 26

Reduced the number of cracks within the material
Form the material under compression

Question 27

When does rubber obey Hooke’s law

Answer 27

When the force is small

Question 28

Why is more work done when stretching rubber than when contracting it

Answer 28

Due to the extra energy used to stretch being transferred to vibrational energy in the rubber molecules. The vibrations re tangle the chains.