Chapter 6. Deformation Of Solids

Question 1

What is density?

Answer 1

Mass per unit volume
density = m(kg)/v(m³)

Question 2

State Hooke’s law

Answer 2

The stretching force is directly proportional to the extension provided the elastic lmit has not been exceeded
F=kx
x= stretched length - original length

Question 3

Note

Answer 3

Before the material exceeds elastic limit it behaves elastically, it goes back to the original size and shape
On a Force-extension graph the gradient will be equal to k, the spring constant
The area under the graph is the elastic potential energy provided the elastic limit has not been exceeded

Question 4

Note

Answer 4

Materials that permanently deform are plastic
Plastic materials are ductile-can be drawn out into wires and malleable-they can be hammered into sheets

Question 5

Note

Answer 5

malleable materials like leadare tough meaning they gradually absorb a lot of energy before they snap
the opposite is true fro brittle materials like glass
a thinner material has greater tensile stress

Question 6

What is stress?

Answer 6

Force per unit cross-sectional area
stress(Pa) = F(N)/A(m²)

Question 7

What is ultimate tensile stress?

Answer 7

A measure of the strength of an omaterial
if stress is less than the ultimate tensile stress of a material then it will hold

Question 8

What is tensile strain?

Answer 8

A fraction of the original length by which a material stretches
strain = extension(m) / original length(m)

Question 9

What is the young modulous of elasticity?

Answer 9

Stress per unit strain
On a stress-strain graph the gradient is the young modulous
E(Pa) = stress(Pa) / strain
A measure of how difficult it is to change the shape of a material
The area under the graph is energy stored per unit volume
The larger the area the tougher the material(does not reach breaking point easily)

Question 10

Properties of materials

Answer 10

Stiff - hard to stretch
flexible - easy to stretch
elastic - goes back to original shape and size
plastic - does not retain its original shape and size
tough - hard to snap(usually goes under plastic deformation)
brittle - breaks easily

Question 11

Note

Answer 11

For springs in parallel, the extension of each spring is the same and the weight of the load is equal to the sum of all the forces on each spring
In series the tension in each spring is the same and equals the weight and the total extension is equal to the sum of all the extensions of each spring

Question 12

What is the proportional limit?

Answer 12

The maximum load or tension in the wire when the extension is proportionalto the load. The gradient equals the young modulous.

Question 13

What is the elastic limit?

Answer 13

The maximum load or tension in the wire which the wire can experience and still gain its original shape and size after the load is removed. The limit of proportionality comes first on a stress strain graph. Beyond this there’s plastic deformation now. Note elastic does not mean the extension is proportional to the load.

Question 14

What is the yield point-Y1?

Answer 14

This is where the material weakens temporarily. Beyong Y2 a small increase in stress causes a large increase in strain as the wire undergoes plastic deformation

Question 15

What is the ultimate tensile stress?

Answer 15

The maximum stress which can be applied. Beyond this the wire loses its strength, extends and becomes narrower until it breaks at the breaking point.

Question 16

Note

Answer 16

Make sure you know the stress-strain graph well with the following points
-limit of proportionality
-elastic limit
-yield point 1 and 2
-ultimate tensile stress and breaking point

Question 17

What is strain energy(elastic potential)?

Answer 17

Strain energy is the ability of an object to do work as a result of the change of shape of the object.