Section 4: Materials

Question 1

Scalar examples

Answer 1

Length/distance, speed, mass, temperature, time, energy

Question 2

Vector examples

Answer 2

Displacement, velocity, force(including weight), acceleration, momentum

Question 3

Difference between a scalar and a vector?

Answer 3

Vectors have a direction, scalars don’t have a direction.

Question 4

Define the term ‘density’

Answer 4

Mass per unit volume

Question 5

Give the formula for density

Answer 5

p=m/V
p-Density in Kgmˉ³
m-Mass in Kg
V-Volume in M³

Question 6

Give the formula for Hooke’s law

Answer 6

F=kΔL
F-Force in N
k-Stiffness constant in Nm^-1
ΔL-Extension in m

Question 7

What is meant by the elastic limit of a material?

Answer 7

The force (or load) beyond which a material will be permanently stretched

Question 8

What is meant by the limit of proportionality for a material?

Answer 8

The force beyond which force is no longer proportional to extension

Question 9

What does it mean if a material is deforming elastically?

Answer 9

A material that is deforming elastically returns to its original shape once the forces acting on it are removed

Question 10

What does it mean if a material is deforming plastically?

Answer 10

A material that is deforming plastically is permanently stretched once the forces acting on it are removed

Question 11

What is meant by tensile stress?

Answer 11

The force applied divided by the cross-sectional area

Question 12

What is meant by tensile strain?

Answer 12

The change in length divided by the original length of the material

Question 13

What is meant by breaking stress?

Answer 13

The smallest stress that’s enough to break a material

Question 14

Explain how you would find he elastic strain stored by a stretched material using its force-extension graph

Answer 14

Work out the area underneath the curve up to the extension required

Question 15

Briefly describe how energy is conserved as a stretched wire undergoes plastic deformation

Answer 15

Some energy is transferred to elastic potential energy, but some of the kinetic energy is used to separate the atoms and is dissipated as heat

Question 16

Describe the energy changes in a spring as it is stretched and released

Answer 16

When a spring is stretched, kinetic energy is transferred to elastic strain energy and stored in the spring. When the spring is released the elastic strain energy is transferred to kinetic energy again

Question 17

Give the derived formula of the Young modulus

Answer 17

E= tensile stress/tensile strain
= F÷A/ΔL÷L
= FL/AΔL
E-Young modulus in Nmˉ²
F-Force in N
A-Cross-sectional area in m²
ΔL-Extension in m
L-Initial length in m

Question 18

What does the gradient of a stress-strain graph tell you?

Answer 18

The Youngs modulus of the material tested

Question 19

What does the area under a stress-strain graph give you?

Answer 19

The (strain) energy per unit volume stored in the material

Question 20

What is the yield point of a material?

Answer 20

The stress at which a large amount of plastic deformation takes place with a constant or reduced load

Question 21

What is shown by the area between the loading and unloading lines on a force-extension graph of a material undergoing plastic deformation?

Answer 21

The area between the two lines is the work done to permanently deform the material

Question 22

What is a brittle material?

Answer 22

One which doesn’t deform plastically, but snaps when the stress on it reaches a certain point

Question 23

Do brittle materials obey Hooke’s law? Explain your answer.

Answer 23

Yes. The stress-strain graph for a brittle material is a straight line, which shows it obeys Hooke’s law

Question 24

Give an example of a brittle material.

Answer 24

Ceramics

Question 25

What is a brittle fracture?

Answer 25

When a stress applied to a brittle material causes tiny cracks at the material’s surface to get bigger and bigger until the material completely breaks

Question 26

Give the formula for calculating stress

Answer 26

Stress=F/A
Stress-Nmˉ² or Pa (pascals)
F-Force in N
A-cross-sectional area in m²

Question 27

Give the formula for calculating strain

Answer 27

Strain=ΔL/L
Strain-No units as it is a ratio***
ΔL-Extension in m
L-Initial length in m

Question 28

When are a pair of forces tensile?

Answer 28

If they stretch an object

Question 29

When are a pair of forces compressive?

Answer 29

If they squash an object

Question 30

Give the derived formula for work done when an object obeys Hooke’s law

Answer 30

Work done=½FxΔL
E=½FΔL
=½K(ΔL)²

Question 31

How is the work done energy stored in an elastic deformation?

Answer 31

Elastic strain energy in the material

Question 32

Give the overall equation for the energy changes in an oscillating spring

Answer 32

Change in kinetic energy= Change in potential energy