Materials

Question 1

What is Hookes Law?

Answer 1

Extension (Change in L) is directly proportional to force applied (F) given that the environmental conditions are kept constant and the limit of proportionality hasn’t been surpassed.

F= k * Delta L
(k =spring constant)
(Delta L = change in length)

Question 2

What is Meant by Tensile Stress?

Answer 2

The Force Applied per Cross sectional Area
F/Area

Nm^-2

Question 3

What is meant by Tensile Strain?

Answer 3

What proportion of a materials original length it has extended by

A Measure of how the material stretches

Delta L / L

Question 4

What is the Difference Between Plastic and Elastic Deformation

Answer 4

Elastic: When a force is removed the object will return to its original shape

PlasticL After the force is removed the object will not return to its original shape

Question 5

What is Breaking Stress

Answer 5

The Maximum Stress a material can endure before fracturing

Question 6

What is meant when a material is described as brittle

Answer 6

A Material deforms elastically, but breaks immediately when the stress reaches a certain value (elastic limit)

Question 7

What is Elastic Limit

Answer 7

The point which stress causes a material to exhibit plastic behaviour

If surpassed the material will begin to undergo plastic deformation

Question 8

What does the area be near a force-extension graph represent?

Answer 8

The work done to deform the material
WD = 0.5F Delta L

Question 9

How do you calculate Elastic Strain Energy

Answer 9

0.5k(Delta L)^2

Question 10

What is Young’s modulus

Answer 10

Tensile Stress/Tensile Strain
Describes a materials elasticity

Question 11

How do you find Young’s modulus from a stress strain graph

Answer 11

The Gradient

Question 12

On a force Extension graph what is the significance of the unloading line?

Answer 12

Unloading line tells you if the material has undergone plastic deformation

If Unloading line doesn’t go through the origin then the material is permanently stretched

Question 13

How does a force extension graph show if a material is obeying hookes law?

Answer 13

Constant Gradient Line through the origin

F and Delta L are directly proportional

Question 14

What is the limit of proportionality

Answer 14

The point after which Hookes Law is no longer obeyed

Shown by a curve forming after a constant gradient on a force-extension graph

Question 15

Describe & explain the relationship between loading and unloading lines on a plastically deformed material

Answer 15

Parallel but not identical

The Spring constant k hasn’t changed
The forces between the atoms are the same when loading and unloading

But the material is permanently deformed so when no force is applied it is not at its original length

Question 16

Under Plastic deformation why isn’t all the work done stored as elastic strain energy

Answer 16

Work is done to move the atoms apart.
Energy as a result is dissipated as heat energy

Question 17

Explain one uses for the knowledge of plastic and elastic deformation

Answer 17

. Crumple Zones deform plastically to minimise kinetic energy transfer to the passengers

. seat belts stretch to convert the passengers kinetic energy to elastic strain energy

Question 18

Outline the Energy Changes when a spring with a fixed top is pulled down and released

Answer 18

Elastic Strain (WD pulling down) —> Kinetic (As Spring is released) —> Gravitational (as spring rises)

Question 19

How do you find the ultimate tensile stress

Answer 19

Highest point on a stress strain graph

Maximum stress a material can withstand before fracture

Question 20

What is meant when a material is described as ductile

Answer 20

A Ductile material can withstand a larger amounts of stress when deforming plastically, before fracturing after the elastic limit has been reached

Question 21

How do you calculate the spring constant for 2 spring connected in series?

Answer 21

1) Calculate extension of all springs and equate it to f/K
2) Total Extension = Extension in first + Extension in second
3) Sub in f/K
4) as the f applied will be the same for both you can divide through it
1/K = 1/(first spring K) + 1/(second spring K)

Question 22

How do you calculate the spring constant for 2 spring connected in parallel?

Answer 22

1) Force total = Force(first Spring) + Force(Second Spring)
2)Force total = (K(first) + K(second))* Delta L
3) K Total = (K(first) + K(second)) as F/Delta L = K

Question 23

Describe an Experiment to test young’s modulus (6 Points)

Answer 23

1.) Find the cross-sectional area of the wire. Use a micrometer and measure the diameter of the wire in several places. Then use formula for area of a circle for finding cross sectional area.
2.) Clamp the wire to the bench so you can hang weights off one end, add a marker at suitable distance from the clamp point
3.) Measure the distance between the fixed end of the wire and the marker - this is the un-stretched length
4.) Then if you increase the weight, the wire stretches and the marker moves
5.) Increase the weight hung off of the end of the wire in steps, recording the marker reading each time - the extension is the difference between this reading and the un-stretched length.
6.) You can use your results from this experiment to calculate the stress and strain of the wire and plot a stress-strain curve

Question 24

Why do Armco Barriers damage crash test dummies less then concrete walls.

Answer 24

Armco barriers deform plastically, this means the contact time between :) the car and the barrier has increased while the barrier deforms

Impulse = Change in momentum = force * time

Dummies as a result receive a smaller impulse so they incur less damage