MATERIALS

Question 1

What is Hooke’s law

Answer 1

force is directly proportional to extension

Question 2

What is Stress (Pa)

Answer 2

Force per unit cross-sectional area

Question 3

What is Strain (none)

Answer 3

Extension per unit length

Question 4

What is the Young Modulus (Pa)

Answer 4

The ratio between stress and strain for a specific material

Question 5

What is the Ultimate Tensile Strength (P) (also Breaking Stress)

Answer 5

The maximum tensile stress that can be applied to a material before it breaks

Question 6

What is Elastic deformation

Answer 6

The change in shape of a material under stress that is recoverable when the stress is removed - Object returns to original shape.

Question 7

What is Plastic deformation

Answer 7

The change is shape of a material under stress that is not recoverable when the stress is removed- Object does not return to its original shape.

Question 8

What is the equation for tensile/compressive strain?

Answer 8

Extension/original length

Question 9

What is the equation for young modulus

Answer 9

Stress/strain

Question 10

What is the equation for tensile/compressive stress?

Answer 10

Force/Area

Question 11

What is the equation for Density

Answer 11

mass/volume

Question 12

Limit of proportionality

Answer 12

The point beyond where force is no longer directly proportional to extension and the material no longer obeys hookes law

Question 13

Elastic limit

Answer 13

The point beyond which the material will no longer return to its original length and plastic deformation has occured

Question 14

Yield Point

Answer 14

The point beyond which a reduced or fixed value of stress causes large increase in strain

Question 15

Tensile strength

Answer 15

The maximum stress a material can withstand before breaking

Question 16

Breaking stress

Answer 16

The stress at which atoms separate completely and the material breaks.

Question 17

What does the area show in a force extension graph?

Answer 17

The elastic strain energy/work done

Question 18

What does the area show in a stress/strain graph?

Answer 18

The elastic strain energy per unit volume

Question 19

Feature of a force/extension graph?

Answer 19

The graph depends on the dimensions and cross sectional area of the sample used

Question 20

Feature of a stress/strain graph?

Answer 20

The graph will always be the same for the same material regardless of dimensions

Question 21

Volume of a cylinder

Answer 21

V=πr²h

Question 22

Volume of a sphere

Answer 22

4/3πr³

Question 23

Surface area of a sphere

Answer 23

4πr²

Question 24

Stiffness

Answer 24

Low extension for high stresses

Question 25

Hard

Answer 25

Resistant to abrasion and indentation

Question 26

ductile

Answer 26

Can be bent and drawn into wires without losing strength

Question 27

malleable

Answer 27

Capable of being shaped but loses strength

Question 28

Laminar flow

Answer 28

When adjacent layers of fluid flow parallel to one another and do not cross into each other.No abrupt changes in speed or direction.

Question 29

Turbulent flow

Answer 29

When adjacent layers of fluid abruptly change speed or direction and do not flow parallel to one another and do cross into each other producing eddy currents and vortices.

Question 30

Criteria for stokes law to apply

Answer 30

1. Small spherical objects

2. low speeds with laminar flow

Question 31

Determining young’s modulus of a wire:

Answer 31

1. Measure original length of wire using a meter ruler.
2. Measure the extension of a wire using Vernier callipers on a sliding scale or a micrometer for different values of the force applied.
3. Measure the diameter of the wire in three places using a micrometer and take an average. Calculate the cross sectional area of the wire, pi (d/2) ^2.
4. Plot a graph of stress (F/A) against strain (extension/original length).
5. Determine the gradient of the linear region of the graph, this will be the young’s modulus

Question 32

Upthrust

Answer 32

The weight of fluid displaced

Question 33

Terminal velocity

Answer 33

The constant velocity reached when an object is falling through a fluid and there is no resultant force acting upon it

Question 34

What is Stoke’s drag force

Answer 34

1. F = 6πrηv where:
 r= radius of the sphere
 v= terminal velocity
 η= viscosity of the fluid 
– this force acts on an object in the opposite direction to its velocity and only applies for small spheres, travelling slowly through a fluid with laminar flow.

Question 35

Determining the viscosity of a fluid

Do this on paper

Answer 35

1. Measure the diameter of a small sphere using a micrometer in three places and take an average.
2. Drop the ball through a measuring cylinder filled with the fluid and marked with two rubber
   bands – the first of which should be placed about 10cm below the fluid level as the ball will be at terminal velocity there. The second should be the maximum possible distance away from the first.
3. Measure the time taken for the ball to fall between the two rubber bands, t.
4. Calculate the terminal velocity of the ball, v = s/t where s is the distance between the bands.
5. Repeat for balls with different diameters.
6. Measure the mass of each ball using a mass balance.
7. Calculate the density of the ball by using the equation ρobject = mass / volume = m /4/3 πr3 .
8. Calculate the density of the fluid by measuring the mass of a known volume of fluid and dividing mass/volume.
9. For an object at terminal velocity obeying Stokes’ law, W = U + F, so 4/3 πr3 ρobject g = 4/3 πr3 ρfluid g + 6πηrv.
10. Rearranging this, v = 4/3 πgr2 (ρfluid - ρobject) / (6πη).
11. Plot a graph of r2 on the x axis and v on the y axis and the gradient of the graph should be 4/3 πg(ρfluid - ρobject) /(6πη).
12. Determine the gradient of the line of best fit and η = 4/3 πg(ρfluid - ρobject) /(6π x gradient)

Question 36

Explain why the wire used when measuring the Young Modulus of copper in a school laboratory is long and thin

Answer 36

1. As each material has a fixed young modulus (E) and if we consider a fixed load (F).
2. delta x=Fx/AE
3. This equation means that the longer the original length (x), the higher the extension ( when E,F,A all remain fixed)
4. It also means that the thinner the wire and hence the smaller the cross sectional area the higher the extension (delta x) when F,E,x all remain fixed.
5. Therefore the extension measured can be higher, which produces a result with a lower percentage uncertainty in delta x, as percentage uncertainty= 100 x absolute uncertainty/value.
6. A lower percentage uncertainty means an increased accuracy as the final result will be closer to the true value.’