Unit 2 Section 2 Materials

Question 0

What does k mean in Hooke’s law?

Answer 0

The stiffness constant of the spring.

Question 1

What does Hooke’s law say?

Answer 1

Extension is proportional to force.

Question 2

What does Hooke’s law apply to?

Answer 2

Springs and stretched wire.

Question 3

What is a tensile force?

Answer 3

When something is stretched.

Question 4

In springs, what is the K value for compression equal to?

Answer 4

The value when stretching. This isn’t the same for all materials.

Question 5

What is the elastic limit?

Answer 5

If the load is increased past the elastic limit, the material will be permanently stretched. Plastic stretching will occur.

Question 6

What are the two types of stretch?

Answer 6

Elastic or plastic.

Question 7

What is an elastic deformation? Explain it at an atomic level.

Answer 7

The material returns to the original shape when the forces are removed.
When the material is put under tension, the atoms are pulled apart.
They can move small distances relative to the equilibrium positions, without actually changing position in the material.
When the load is removed the atoms return to their equilibrium distance apart.

Question 8

In a metal when Hooke’s law is obeyed, what type of deformation is it?

Answer 8

Elastic deformation.

Question 9

What is plastic deformation?

Answer 9

The material will be permanently stretched.
The atoms in the material move position relative to each other and when the load is lifted they don’t return to the original positions.

Question 10

When a metal is stretched past its elastic limit, what deformation does it show?

Answer 10

Plastic deformation.

Question 11

How is the young modulus found?

Answer 11

Tensile strength ÷ tensile strain.

Question 12

What are units for the young modulus?

Answer 12

Nm²

Question 13

What type of wire should be used when testing the young modulus of a material?

Answer 13

The thinnest wire will stretch the most.

However it is more likely to snap.

Question 14

If you play a graph of stress against startin, how do you find the young modulus?

Answer 14

It is the gradient.

Question 15

How is tensile stress found?

Answer 15

force applied ÷ cross sectional area.

Question 16

What are the units for stress?

Answer 16

Nm^-2 or pascals

Question 17

Define tensile strain.

Answer 17

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

Question 18

What happens when a material feels too much force and breaks?

Answer 18

The stress increases and the atoms are pulled away from each other. Eventually the stress becomes so strong the atoms fall apart completely and it breaks.
The stress where this happens is the braking stress.

Question 19

What is the UTS?

Answer 19

Ultimate tensile stress. The maximum stress a material can withstand.

Question 20

What is the elastic strain energy?

Answer 20

The energy stored in a stretched material. Before the elastic limit, all the work done to the material becomes energy stored in it.

Question 21

How can elastic strain energy be found with a graph?

Answer 21

Draw a graph of force against extension. The stored energy is the area under the graph.

Question 22

How can the energy stored in a material be calculated?

Answer 22

Work done=0.5 X force X extension.

word done= 0.5 X spring constant X extension²

Question 23

Why do the equations for finding the work done to a wire only work when the force is below the elastic limit?

Answer 23

If the limit is exceeded then work is done to move the atoms about.

Question 24

How do you calculate the strain energy per unit volume of a loaded wire?

Answer 24

0.5 X stress X strain.
Or 0.5 X force X extension
Or E= 0.5 X K X extension²

Question 25

What is a brittle material?

Answer 25

A material that won’t deform plastically when a force is applied but will snap suddenly when the force gets big enough.

Question 26

What is the structure of a rigid material?

Answer 26

They are crystalline or polycrystalline. There are many different grains that all line up in different directions.All the atoms are tightly bonded to one another meaning they are stiff. It is a giant rigid structure, thus brittle.

Question 27

What happens when a brittle structure has a force applied?

Answer 27

Tiny cracks will appear in the materials surface that get bigger and bigger until the material breaks.
They grow because they have rigid structures.

Question 28

How do you find the area under a curve?

Answer 28

Count the squares.

Question 29

What is the spring constant for springs in parallel?

Answer 29

The spring constant of the 2 springs added.

Question 30

How is the effective spring constant for springs in series found?

Answer 30

Imagine if they were cells in parallel and use that equation.

Question 31

When measuring the young modulus of a material, how can accuracy be improved?

Answer 31

Long thin wire. Gives larger extension per unit force therefor percentage uncertainty reduced.
Control wire used so temp changes do not impact results.
Measure diameter at serval places and find average.

Question 32

When a material is stretched past the electric limit, where does some of the energy go?

Answer 32

Lost as heat.