Chapter 6 - Materials

Question 1

What is Hooke’s Law?

Answer 1

Hooke’s Law states that the extension in metal wire is proportional to the force applied.
F = Kx
Where F is the force, x is the extension and K is the spring constant.

Question 2

How does Hooke’s Law apply to springs?

Answer 2

The extension and the compression of a spring is proportional to the force applied.

Question 3

When does a material stop obeying Hooke’s Law?

Answer 3

Hooke’s Law applies to metal wire and spring until the limit of proportionality is met, this is when a force extension graph stops being linear.
The elastic limit is the maximum amount of force that can be applied that will not permanently deform the material.

Question 4

How do springs in series and parallel differ?

Answer 4

In Series: 
1/Spring Constant of system = Sum of: 1/Spring Constant of Individual Spring
1/Ksystem = 1/K1 + 1/K2  
More extension that just 1 spring.
In Parallel:
Spring Constant of system = Sum of Spring Constant of individual Springs
Ksystem = K1 + K2
Less extension than just 1 spring.

Question 5

What is the difference between elastic and plastic deformation?

Answer 5

Elastic deformation - Material will return to its original shape is the load is removed.
Plastic deformation - Material is permanently deformed and won’t return to its original shape.

Question 6

How can we investigate the spring constant?

Answer 6

• Hang different weights from a spring, and record the extension.
• Plot a force against extension graph.
• The gradient is the spring constant.

Question 7

Define Tensile Stress.

Answer 7

Tensile Stress is defined as the force applied per unit area.
𝞂 = F / A
Units = N㎡ or Pa

Question 8

Define Tensile Strain.

Answer 8

Tensile Strain is defined as ratio of change in length. Extension divided by the original length.
𝜀 = x / l
There are no units.

Question 9

What are the Key Features of a Force Extension Graph?

Answer 9

• The Gradient is the force Constant.
• The Area under the graph is the elastic potential energy.
• Linear part of the graph is where the material obeys Hooke’s Law.

Question 10

Define the Young’s Modulus

Answer 10

Youngs Modulus = Stress / Strain = Force x Length / Extension x CS Area
E = 𝜎 / 𝜀
Its units are N㎡ or Pa

Question 11

How can you Practically investigate Young’s Modulus?

Answer 11

• Set up a wire fixed at one end and the other end over a smooth pulley with a weight suspended on it.
• Measure: the length of the wire, the CS area of the wire, the weight on the wire at the corresponding extensions.
• Draw Graph of Stress over Strain
• Gradient is the Young’s Modulus.

Question 12

What are the Key Features of a Stress Stain graph?

Answer 12

• Gradient is the Young’s Modulus.
• For the Linear part of the graph, the area under the Graph is the energy per unit volume.
• The UTS is the maximum stress value on the graph before the material will breaks.
• The rough shape is linear until the limit of proportionality, the gradient decreases, reaching a maximum (UTS). Then the gradient continues to decrease until the material breaks.

Question 13

What does a Stress Stain graph for a brittle material look like?

Answer 13

The graph will only go to the limit of proportionality, then will snap. So the graph is only linear.

Question 14

What do the Loading and Unloading curves on a Stress Strain graph look like for Rubber?

Answer 14

Rubber: Rubber is Elastic unless broken.
The loading and unloading curves are different as the energy released when unloaded is less than the work done to stretch the rubber.
The area between the curve is the energy lost to thermal energy.

Question 15

What do the Loading and Unloading Curves on a Stress Strain graph look like for Polythene?

Answer 15

Loading: Steep Incline, then a small sudden curved drop. Then a straight line.
Unloading: Sudden Steep Straight line down.